Rewatching this once again a few months back, I changed my mind on this
Forest is really weird about sex. I used to think it was because his mom was always bringing men over and his nerodivergence, but his reaction to it is a trauma response - he dissociates whenever people make a move on him.
There’s a lot of hints that Jenny was sexually abused by her step dad, but in the beginning forests intro about him can be interpreted as him having done something to forest as well
In that context, think about their first kiss in her dorm - he gets weird and she freaks out. I assumed it was because she felt he wasn’t able to consent, but maybe she freaked out because she realized she was triggering a trauma response and felt sick to her stomach
Then when they have sex, it’s forced and uncomfortable for both of them. “I’ll show you what love is” He did it just to fulfill her needs so she’d stay - but obviously that’s not something she’s ok with
In that context, it’s not that Jenny doesn’t love him, it’s that he would be hurt to be sexual with her, and if she’s around he’d be hurt if she was with others