Given the dysfunction between the two at the start of the game and the inner monologues we've seen from Kaija, I suspect the sex wasn't that great overall. There were probably a few occasions where the stars aligned and the genuine affection they had for each other took over, but most of the time I think it would have been a very uncomfortable, self-conscious business where one or the other was caught up in their own head (which would then make the situation even worse).
I do hope we'll get a chance to explore that side of the relationship soon (for those who keep it going), but it sounds like we'll have to wait until at least Chapter 6.