Sex with Exes

Okay so I had this thing with this guy this summer and I wanted to have sex with him so bad, and the sexual tension is just there it's ridiculous and it would be so fun for us. But I have a boyfriend now, who I love dearly, I would never cheat on him. ever. like not even a thought. but sometimes I think about if I'll ever be single, how I'd have sex with the other guy. is this wrong??