If he tries anything, I'm going to say no to this guy I like because I should, right?

[Mandy]

My program finishes in a few weeks and when it does, I'm leaving this country. I met this guy a few weeks ago, and we clicked instantly. There's such a huge mutual attraction on both ends that it's unreal. He wants to see me on Sunday for some coffee.
But, the plot thickens. All over his Facebook profile are "I love you" images from his girlfriend and him replying with things like "I love you too, princess". Plus, one of his images he sent her came up on my newsfeed yesterday talking about how he could see them getting married and living together in the future.
I cannot be that girl that ruins or puts a damper on this relationship. Even though we would have been great for each other if he was single and I wasn't leaving, I have to respect this girl I don't know even though he hasn't mentioned her.
I have friends (devils on my shoulder) saying I should "leave my morals back home", go for it, and see what happens, but I can't and won't. I'm not going to initiate anything, but I wouldn't be surprised if he did. If that happens, I'm going to tell him that I know about his girl.
My reasoning is usually a blurry battle between my heart and head, and I just want to confirm that my thought process is sane. If you disagree with this, why? Thanks!
@Brandon: Exactly. Thanks for that.

[Brandon]

By what you are saying here, he is the main person in his life and you, unfortunately, are the "other woman". That means that he doesn't love you..by what he writes over Facebook, he is either devoted to her or he is a huge liar. And he is not such a great person if he is a cheater. Is that someone that you want to be with?