Weigh your options....is it more important to have sex with other people, or is it more important to have a relationship with this guy? It's pretty obvious that you can't have both, and I dont' think it's fair to ask him to be OK with it when he's not. Not to say that you're a bad person or anything (because you're not!) But just to toss it out there, I'd be really offended if my boyfriend asked me more than once (and I said no!) to be OK with him having sex with another person, man or woman.
Really, really, just think it through. And like the others alluded to...you're young. It's totally understandable to want to have sex with other people, not just one person forever. Seriously, just sit down with yourself and try to figure out which one will make you happier. Both options are going to make you deny yourself something...you just gotta figure out your priorities.
And, seriously hun, if you think that 'maybe i'd rather be single and open to do whatever with whomever', it's a pretty good indicator that you *should* be single, at least for right now.
I'm rambling. But good luck, either way!
(And I hope I didn't offend you or anything!)