Are there guidelines to these kind of things?!

So there's this guy. Haha, isn't there always? But anyway, I met him my first week of college, freshman year. He was in my Intro to Visual Arts class (a total bullshit, waste of time and money class, if you ask me). I met him the first time I went to that class. I asked him for directions and he asked me if I wanted to smoke. Ummm....of course I wanted to. I didn't go smoke down with him by myself though, I had a friend from high school with me who had that class too. No, that would have been a horrible idea to go alone, considering I had a boyfriend. I never wanted to do anything to make myself seem like a bad  girlfriend. We smoked, we talked, he took us back to our dorms, and then after that we sat next to each other in class each time I went, which was very rarely and mainly on exam days. You see, I had this class on Tuesday and Thursday nights. From 7 to 9. Every college student knows what goes down on Thursdays. I didn't have time to go to that insignificant class on a Thursday night when I had so much other shit to do like, go to dinner with the girls, go to the gym for an hour, then go back to my dorm and shower and get ready for the night's festivities. But getting to my point. I've known this guy for a while now, since I'm already halfway through my fourth semester of college. We used to party a lot too, but now we barely see each other and talk. Well, tonight, after I got back from the gym and after I had totally humiliated myself in front of a group of guys at the hot tub, I decided to see if this guy wanted to come smoke, since after all, he's a dealer. He has always given me and my friends pretty good deals on pot. Not to mention, he has some pretty dank shit. One hit gets me high, which in turn saves me some money since I'm not a frequent smoker. I can make an eighter last a month and a half, at the very least. So the guy comes over, smokes me down, didn't make me pay a dime for the bowls we smoked. I hadn't planned on this, by the way. I was actually going to go bowl for bowl with him, but he insisted I kept my shit. What a nice guy, right? The whole time he's at my apartment, all I could think about was how I haven't been laid in what seems like fucking years and how attractive and appealing he was looking. Needless to say, absolutely nothing sexual happened between us tonight, but I can't say the thought wasn't on my mind the entire time. The thing that made me want him even more is that he was nice enough to leave a few small buds with me. A parting gift? I don't know and I don't really fucking care because weed is weed. After he left, I was kicking myself for not making a move, but then this thought cross my mind, "If I had sex with him, would I be considered a prostitute? Would it be like me exchanging sex for weed?" I asked a couple people what they thought and they said that it's definitely not prostitution. But what if we had ended up boning? Would leaving the weed seem like my payment, say if he left it after the act was done and over with? Would it seem like he was paying me up front if he had given me the weed before we had sex? What are the guidelines about these kinda things? Do booze and drugs constitute as the same thing as a guy taking you out to dinner before he rails the shit out of you or buying you breakfast after a crazy night? Is it just that my mind is being stupid and way too over-analytic because I haven't gotten laid or really given any kind of guy contact much thought for a while now? Am I just getting really fucking dumb about guys now?