Why is he always pent up? What can I do?

My fiancé (20m) and me (20f) have been together for 3.5 years now and we love each other but recently he’s been really pent up. At first it was because of work because we found out work causes him to be frustrated and that ends up turning into sexual tension. Recently however he’s been doing a lot, things like groping my ass around friends, trying to kiss my neck and constantly telling me how hard he is for me, I get that he’s attracted to me and all that but it’s getting annoying and I tell him that it’s bothering me because him being overly handsy isn’t helping anything. Telling him to stop however isn’t doing anything though because he’ll just end up doing it again anyway, like he’ll stop for the day but then continue a day or two later. It also makes him feel guilty which then leads to him not wanting to be affectionate at all so no hugs or kisses. I just have an issue with all the sexual advances. We talked about it in therapy but he’s still not seeming to catch the memo. I just want to be with him and do normal things like cuddle and watch a movie. He’s also offered to take care of himself but that leads to him being in the bathroom for an hour or two because it takes him longer to take care of it with just his hand.

Why is he always pent up? What can I do?
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