Asexual

Okay, so I'm asexual and I have had sex in the past because my SO wanted to but I'm with this new guy, we've been together for a while, and we're pretty open about everything and he asked if we would ever have sex. Now I don't feel the urge to have sex, never have never will. Plus I think sex is kinda gross (no offense to anyone please don't hate) but he wants to and I'm not sure if I can. I mean physically I can but I've just had a bad past and it makes me hella uncomfortable to even think about it. He doesn't know that particular part of my past and I'm not sure if I should tell him because if i do maybe he'll understand but at the same time he may think I'm a freak and leave me. So what should I do?