Boyfriend Conflict!!!?

So to make this story short and sweet, I know what my boyfriend's FB password is. And I guess this was wrong on my part, but I logged in and looked around. I came across some messages with a girl that I know from school...and she wanted weed. And when it comes to that, it's just something I hate. That he is involved with that sometimes, but this time to make matters worse he is talking to this girl about giving her weed. I could sense a bit of a flirty vibe from what he wrote and the conversation went on to talk about ,how she didn't have any money in exchange if he did give her any weed, and what seemed to be joking around on his part saying, that she could give him head in return and they could meet at a pool nearby and he could smoke her up...didn't seem so joking anymore. She kept saying "you have a gf not going to do it" and so on, and he said I know that it's not really an option. And anyway the conversation ended by her, continuing to say how she had absolutely no money and he said "the other option is looking better huh?? :)" and she once again said "you have a girlfriend so that eliminates that " and then he said "secrets". What is that you know? That he will keep it from me? Well,despite this whole deal, I really want to confront him about it, but I know it was wrong on my part to be snooping on his fb. The worst part I think...is that I had a mindset on how I was really apposed to weed,and I told him how much I hated it. Later on this summer, I got more of an open mind, and asked him about it...but he told me how good it was that I didn't and all that, but truly I think it's just so I wont like it, and smoke with other guys, even though he can smoke with girls. He told me finally, "you are better than that come on, if you don't start, I will stop. Promise" and then he goes on with this. UGH. I Just dont know what to do...help please.