Well actually things went bad in the beginning but then we made up and all was well for a few months and then she broke things off because of something else. The story with this girl is a long and sad one and I'm not gonna tell it all here for sure. But one thing I can say is that it makes me super sad because they've been together for five years now and they are 100% not happy. They fight all the time (minimum three times a week), rarely have sex (less than once a month - to the point that she visibly gags when he suggests it), she lives in his apartment but he still lives with his parents and has been postponing moving in for 10months, they took a two week trip together and five days into it she offered to pay for a friend to join them because she couldn't stand to be alone with him anymore.


Maybe she is aware of his reputation, maybe she doesn't mind. But I'm more concerned with what you would go through, and you yourself are telling me you wouldn't want to put another woman through this sort of thing, later telling me you felt lied to.
Tell him this, "John, I don't want to ruin your chances with Jennifer, date her and be happy. If things don't work out with her you've got my number and make sure to use it as sex with you is the fucking best" (trust me he'll enjoy the last bit and will make it easier to swallow it).

As for this other relationship you mentioned, can't really say for sure. If you two used to be FWBs maybe he truly likes to sleep with you but doesn't want to get too serious, maybe he actually likes you but doesn't know how to bring it up/how you would react. Say you told him repeatedly you don't want a relationship and he does, this could be a way to have some sort of relationship with you?

Feel free to shoot a PM if you need someone to talk to.