Relationship Advice 101:
Lesson 1:
Never, ever, ever, ever assume anything.
Golem, I feel* that is was unfair of you to factor in what your girlfriend did before you guys were official into your relationship at the time. You guys weren't dating, and you had no "claim to her" as they say. Sure you were courting, canoodling, fucking around, what have you, but its unfair to assume that just because you want someone to be exclusive with you that they are going to be.
Jmrz, if he is not ready for a Serious Relationship right now, I don't think it'd be too horribly unfair for you to request that he not do what he is doing with you with anyone else. If you are comfortable with what you guys are doing, I wouldn't worry too much about the future. But you say you'd like to see him long term? Well, basically, you are going to have to talk to him. You're going to have to let him know how you feel about things. Find out why he is opposed to a serious relationship first, though.
Keep in mind that a lot of people get scared off when you keep bringing up "when are we gonna take things to the next level???" Maybe he just isn't ready for commitment. I'm not saying to super freak out about this, but just be prepared for negative reactions.
Or maybe he will be super relieved and say "I was waiting for you to say something! As soon as I said I wasn't ready for a serious relationship I realized I was! But felt silly and needy bringing it up again.
*"feel" - as in, this is my opinion.