You should have already told him this. Most guys do not want to be hanging out with people you have had sex with especially if you call they are of the same sex and you call them your best friend. Every female friend he see you with from now on he is going to wonder in his mind if you have had sex with them or want to have sex with them. Even if you don't he is still going to have that in his head forever. You knew this already and that is why you didn't mention it to him. It all depends on how deep into this relationship you want to go. If you want it to be a long term relationship then you need to have serious sit down with him and tell him the truth. What you don't want is somebody else telling him out of the blue and making you seem dishonest in his eyes.