Most Helpful Guy
It's not cheating and you did nothing wrong but you should tell him, he will find out eventually anyway and finding out from you upfront will be MUCH better then hearing about it some other way.
The reason you don't want to tell him is because you're afraid he will think he is your second choice and that you will walk out on him to have sex with other men when things get hard, even your guy friends that you're not attracted too.
The thing is though is that is a pretty big part of who you are and if he can;t accept that about you then you will never stop being afraid of loosing him and will never be able to to let go and love each other fully.
Most Helpful Girl
As someone who's been in your position but on the other side of it (my ex was the one sleeping around), I say if he's not caring to ask you about it, I don't see why you should volunteer the information. Why? Because once you're single, you're no longer entitled to know one another's personal business. For all you know, he could've been doing the same thing. Point is, you were both clearly broken up so you did nothing wrong by trying to move on.
Now, if during that time you were still talking to him in a way you feel could be taken as leading him on, or if he's outright asked you if you did, then I feel like that's where honesty is typically the best policy. My ex didn't want to tell me about it at first (and looking back, I realize I shouldn't have asked and just left it alone) but he eventually did tell me the truth, and what hurt like hell was not that he had hooked up with other people but that he had lied about it.