Am I wrong to be upset at my g/f about this?

Now I’ve never considered myself an overbearing boyfriend. I’ve never minded if a g/f was still friends with an X (regardless of how close they were then and during our time together), I don’t mind a g/f going out without me, or with other guys. I don’t mind flirting, etc.

Well my current g/f lives about 3 hours away, and as you might guess, it’s difficult. Some 2 or so months ago she went out with a group of people. She spent much of the night flirting with one of them (piggy back ride, talking a lot with just him, etc), which doesn’t bother me at all. But what did bother me was that at the end of the night, in what I imagine was a drunken state, he slapped my g/f’s ass. She didn’t like it, but it ended up sparking a big fight between she and I somehow the next day (some other stuff was involved but that sparked it).

Well fast forward to now. For a long while she didn’t really talk to this guy at all. Well a few weeks ago she ends up talking to him again, and now they’re friends or something. To the point where a week or so ago she ended up staying up til 5am talking to him in some common area.

I got pretty pissed off as you might guess. Now I’m not one to tell her who she can and can’t hang out with (which is immediately how she took it). But she can’t seem to understand why I don’t like this guy, and am not a fan of her hanging out with him.

Am I being unreasonable/stupid/whatever here?

Sorry it disappeared, some overzealous mod made me change the title to be “more specific”