Momo is the most aggressive of the Deviluke sisters. Lala is confident through her peppy personality and emotional ignorance, while Nana simply doesn't want to entertain the idea of being intimate with you. They keep fighting in the corner as Momo repeatedly claims your mouth with deep, tongue-filled kisses. There is no hesitation or fear in her actions, she's been waiting to do this for some time and isn't going to miss out for any longer.
Momo isn't just satisfied with playing with your mouth – as her mischievous fingers start to worm their way beneath your clothes and feel your skin. Your pectoral muscles and abdomen are of particular interest to her given that you've been working out almost nonstop for months on end. You've started to turn into a guy who looks like he belongs in an underwear catalogue. Momo is very happy about it. She purrs into your mouth and allows them to dip down south beneath your belt.
Oh, she's really going for it. But before Momo can get a handful of your sausage, there's a cry of outrage from across the bedchamber.
"Hey, what the hell are you two perverts doing over there?" Nana yells.
Momo releases your lips and pouts, "I'm kissing my fiancé, what else?"
Lala redresses herself and finally puts an end to their sisterly duel, "There's nothing to worry about with hubby, Nana. He likes girls in all shapes and sizes!"
"That's not the problem here! Why did you decide that now was the right time to start playing with him?"
Momo tilts her head to the side in confusion, "Why not? This is the private chamber that Lala built so that hubby can sleep with his wives whenever he likes. It's completely soundproof and impenetrable from the outside. Nobody is going to interrupt us."
Momo is plainly just trying to tease her sister by playing dumb. Nana isn't upset about the prospect of being caught or someone spying on you – she's upset because she thinks its weird for her sister to start getting handsy right in front of her. They may come from a world where bigamy is accepted, but that doesn't mean Nana wants to sit there and watch you have sex with her sisters.
"Maybe we shouldn't do something Nana is uncomfortable with," you state, putting your foot down before things break out into another lengthy argument. Momo is disappointed but follows your suggestion. There'll be plenty of time for funny business later without an audience.
Nana is astounded that you aren't diving in with no regard for her comfort, "Hmph. I guess you aren't just a horny beast after all."
"I have a lot more on my mind than that. I've got to worry about stopping these invaders before they conquer our planet, and the relationships I have with the others aren't just based on sex. We go on dates, spend time together, and keep in touch using our interdimensional phones. The foundation of our partnership is about being honest and open about what we want. I don't want to do things that make you uncomfortable."
Lala supports your assertion, "Hubby is really nice. He always asks me and the other girls before doing anything with us."
"Don't think I'm going to jump onto this stupid mattress with you right away. I want to see how you handle being my boyfriend first," Nana declares pridefully, "That means you need to take me out on a date! If you manage that without putting your hands all over me like a zombie, then I'll consider being one of your fiancés."
Momo grins, "And then hubby will be the undisputed ruler of our empire. To think that one man is claiming all three Deviluke sisters..."
She's acting like she has nothing to do with this situation coming about.
"...Anyway – Nana is going to have to get used to this kind of thing. Couples do this to show how much they love each other!"
Nana grimaces, "There are other ways to show you like somebody, like buying them gifts, or taking them on dates!"
"And what happens when you get bored of those things? Are you trying to say that you'll go out with hubby for years but never hold hands with him, or kiss him?"
Momo is veering close to starting another argument here despite your intervention. Nana is just being tsundere. Pushing her buttons isn't going to make her change her mind. Nana needs to be confident that its her own idea or decision. Lala leaps on top of you and cuddles up, "Just let Nana do things at her own pace, Momo!"
Momo pouts, "Fine."
You sit back up from the extremely comfortable mattress that Lala found and offer Nana a reassuring smile, "You don't need to do anything for me. I've never pressured any of my partners into doing things they aren't comfortable with. If you want to go out on a few dates before worrying about stuff like that, then I'm going to do as you ask."
Nana crosses her arms, "Why does he respect me more than you two?"
Momo and Nana glance at each other. "Are we being disrespectful?" Lala inquires.
"Yes! You both do this all the time!" Nana yells, "Do you have any idea how frustrating it is when you both get ahead of yourselves and don't listen to me? Lala is always so invested in her little inventions that she doesn't consider the consequences of using them, and Momo is equally obsessed with trying to manipulate everyone around her!"
Momo shrugs, "Is it a bad thing if everyone is happier for it in the end? We're going to make the biggest, happiest and sexiest harem in the multiverse!" Again, Momo is more excited about the idea than you are – and she isn't the one poised to benefit the most from it. You can see her eyes sparkling with barely concealed joy.
"There's something wrong with you, I swear. We all know that I'm going to be the wife he likes the most anyway!" Nana has had enough arguing for one day, she flops down onto the bed and takes a moment to admire the texture and softness; "Wait a minute, where the heck did you find a nanofoam mattress this big, Lala?"
"It was a lot of work," Lala chirps, "I had to get it custom made at the factory – so I used my allowance to buy a bunch of different smaller ones and had them stitched together to make this super huge bed!"
"Your allowance? How much do you girls get anyway?"
Momo scoffs, "A few billion credits a year – it's basically chump change."
"A few... billion?"
Either their currency is hyperinflated or these three have absolutely no perspective on just how much that really is. You certainly wouldn't describe it as chump change like Momo. You just thought that Lala constructed the entire thing herself. She's more than capable of building all sorts of strange and wonderful inventions, but perhaps building a nice mattress is outside of her field of expertise. Being the daughters of a galactic emperor sure does come with a lot of nice benefits.
You don't have much room to complain about that anymore. Mitsuru is paying you millions of dollars every month, which you wouldn't even need given that getting to be with so many amazing fictional women is part and parcel with the job. It's getting to the point where you can't even think of things to spend your money on. Donating it to charity feels like the only way not to leave it rotting in your offshore bank account.
Nana's tail whips the air irritably, "This is great and all – but what the heck are we going to tell Dad?"
Lala smiles and offers an optimistic take, "I think he'll be super happy that his daughters have found such an amazing and powerful partner!"
"Wait a second. I might be strong by human standards but I don't think I could take on your Father in a fight, even with my armour," you reply, "And I know for a fact that he's keen on having any suitors demonstrate their ability to keep you all safe."
Nana grumbles, "Even though it's a total waste of time. He married our Mother because she was the strongest warrior he could find. There isn't a single lifeform in the entire empire who could pose a risk to us."
"I understand how he feels though. He wants to ensure that all of you are well taken care of. There must be a lot of people out there who have ill intentions for you and the empire as a whole. It's a big responsibility to take care of it."
"Hmph. That stupid heroic sacrifice act you pulled would have been better spent on convincing him!" Nana quips. She's right – that was a showcase of sincerity that you can't fake or describe through words. He'll probably reject it out of hand even if Nana is the one recounting the events to him.
Momo smirks, "Don't worry, I've already put together a plan that we can use to win his approval and confirm our future marriage!"
"I hope it works better than the last one you two idiots came up with," Nana sighs.
"There was nothing wrong with our plan. It was interrupted before we could see how it would have turned out."
Momo is being very defensive of her and Lala's plan, even when it wasn't all that sophisticated in the first place...
Momo has another point to prove before you can call things tied up; "By the way, Lala totally slept with hubby already. Are you saying that we're not going to do it either?"
Nana blushes, "W-What? You want to have..."
"Sex?"
"...Sex with him!?"
Momo sighs, "We're adults, Nana. It's natural that a relationship means you have to do intimate things with your partner eventually."
You interject, "Lala was very eager so I ended up getting dragged along with her whims. You two don't have to feel any pressure about that sort of thing. Spending time with you is already a lot of fun."
Nana hops on her high horse to deliver a stirring speech, "That's what you're saying to lure us into a false sense of security. All men are obsessed with things like sex. They've a bunch of slovenly beasts! Perverts!"
Okay – so maybe you do engage in a lot of sex, but that's because you have a lot of partners. They have needs of their own, and it's a fun way to get closer with them without having to make specific plans for dates or asking around for reservations. Nana is still rather immature; her perspective is coloured by boys who haven't hit eighteen and think they're hot shit sex gods in waiting.
"As I said – there's no need to do any of that if you don't want to."
Lala offers her perspective in her characteristically frank manner, "If you're worried about it hurting or having something big going into your private place, I could be there to make sure that you're ready to handle it!"
Nana scowls, "I don't want you to watch me lose my virginity, Lala."
"Huh? But I'm not going to take any pictures or anything!"
"That's not the problem! I don't want my sister watching me getting taken to pound town!"
"Pound town?" you repeat with a chortle. Nana lets out a strange hawk-like cry as the coarse language slips out without her consent. You've never heard any of your girlfriends or fiancés describe it like that before. You tuck that away into your back pocket for later. Momo, Nana and Lala quickly descend into a tumble of flailing arms and legs as the sisterly debate turns into an all-out wrestling match.
You don't remember them being like this in the manga...
Whatever – you can't expect them to always act like their fictional depictions when they come to your world. They're fully formed people with sides to themselves that only become apparent in different situations. This is what siblings do. They argue and scrap over the smallest things. Though in this case, seeing each other having sex isn't really a small thing at all. None of the other girls have a problem with it, but the sibling relationship may make things a bit more awkward for them.
It doesn't concern you too much. Working out a schedule where you can do things like that without involving the others should be simple enough. All it takes is a little due diligence and consideration for what they want out of this.
For now, you're happy to know that your efforts have paid off in some ways. Nana is finally on board with the harem plan, and her knowledge when it comes to animals will be invaluable to your future efforts. No doubt the idiots you're fighting at present will come up with a scheme involving them, where speaking to them will prove key in fighting back and solving the problem.
The fight ends with Lala claiming victory – standing atop her defeated sisters with a broad smile that is completely at odds with what's actually happening. You sigh and pull out your phone to see who keeps messaging you.
As usual, it's Rias. Unusually, there is no naked image attached. Instead it's a selfie of her and Akeno wearing casual clothes and clearly enjoying their summer together.
"Let's spend time together soon, hubby! Xx"