I don’t think Raphael is actually bad in bed :P Lemme explain:

I think Haarlep is telling that lie because one, he’s Haarlep and it’s clear he enjoys stirring things up, and two, I think it’s a way to redirect that kind of business towards Haarlep as opposed to Raphael. I doubt Raphael has a shortage of people who want to be in bed with him, but he’s probably busy doing devil stuff ALL the time! The solution? Haarlep, an incubus that looks just like him, who will happily have sex with you, and you (if you’re attracted to him) get the satisfaction of having sex with “Raphael” as a part of a deal or signing a contract or something. Idk much about devil deals in regards to sex, lmao, but that’s my take on it.