Having sexual relations with a noble's daughter without the family head's permission is a crime, and this is the punishment. I have to admit, forced slavery is quite often the punishment in this country, but often only a temporary one until the criminal pays off the damages they've caused. Ronan's crime, the daughter's virginity, is valued so highly that he is a slave for life unless his new master decides to set him free of their own volition.
I gotta say, I like Ronan a lot better. He is glancing at us with curiosity and hope, and a bit of perversion towards Ayame as well, something we would have to talk about later, but something I can understand. She is just that gorgeous.