From my one and only experience, a character gets the Loves to Masturbate trait at lv 10 Masturbation addiction. It's a really long, pain-in-the ass process. Ironically, the most tedious part of training it isn't getting the masturbation XP. It's farming the Exposure gems. You need to simultaneously train the Exposure ability and both stats require Exposure gems.
From what I can tell, strong orgasms seem to give the most exposure XP. It might go a little faster if you stimulate all the erogenous zones at once, though I totally understand if you want to avoid deflowering. The Lewd Breast trait (obtained at Rank S Chest technique) apparently raises the Embarrassment (Which I can only hope affects Exposure) parameter faster, but I'm not sure by how much and I can't remember how easy or hard it is to get it.
Good luck, dude. It's probably going to take you a few real life days to get there. If you find anything else that can help, let me know.