The kiss was long and sweet. Xavier held back his desire to kiss her for longer and devour her lips hungrily. One, he didn't want to give Shiro a free show, and two he didn't want Elena to feel uncomfortable.
He realized he had to take it slow. She was still so deeply (but foolishly) in love with his jerk brother. He also only realized his feelings for her recently. He needed time to process everything and made a decision on how he should respond to it. He figured for Elena it would be the same.
Elena would take time to realize her husband was not good for her and she would later have to process her situation and what she should do.
So, Xavier ended the kiss reluctantly. He opened his eyes slowly and found her eyes widened in surprise. Did she think he would peck her on the lips just to answer the Dare from Shiro? She looked surprised but not angry. In fact... her cheeks blushed.
