This is the only thing that makes sense to me. She did take her top off in the bedroom. So, I don't think it's the nudity that bothers her. It's the sex itself.
2 other quick thoughts:
1) She had to have told him multiple times that she would not have sex in this apartment. So, if he made the choice to come home with her anyway (or to come over as he did tonight), then he knew that she didn't want to do it there and he went there hoping to change her mind or because he just wanted to be with her and cuddle, etc.
2) Milana told him in English when he first got there tonight that she had a "flight to catch in the morning" so she wanted to take it "easy tonight". I don't think that she is leaving for good yet, but it is possible.
You make some good points here; However I do have to say that she asked him to go upstairs way before the buzz kill got there and he turned her offer down. I'm not really sure which one of them is hesitant when it comes to being intimate with each other! At any rate, there is a love there it appears and that might account for the NO sex part at the apartment. Who knows!!