At first I was really disappointed with Medea, she is so smart couldn't she think of some excuse and refuse him? Does she really hate him or she actually wanted to do it? Well I got back and looked at old chapters. There is this moment in chapter 38, when Eros comes and Medea says she is tired and asks him to go back but he just ignores it and comes anyway. Medea then says 'He always does as he pleases'. Pretty sure it can also apply to the sex scene. Both Medea and Psyche had to live in each other bodies. Besides kissing with Helio and Eros, Psyche injured Medea body multiple times. Of course it's all fine since she can heal. Practical Medea also may think - Psyche was sleeping with him anyway, it's not like she loses her virginity. Honestly, I don't really understand the author setting - how could they do it before the wedding? But it was weird from ch1, when both go to some honeymoon to celebrate engagement. If you look at it from another perspective if Medea really hate him, she also may feel unpleasant to do it with him. It's just to me feels like Medea doesn't care about pain or her body all that much.