He can be labeled a creep for stopping after she reiterated the point at dinner, but everything up until then?

I'll tell you what I think happened. I think he didn't realize she was in a relationship. Got close to her while she was obliviously trying to make friends. He asked her out for dinner and a movie (which should click in everyone's mind as a date), she said yes. He's enjoying the date, goes a little overboard saying he's enjoying it. She realizes he thinks it's a date and corrects him by telling him she is actually already in a relationship. He, being bit confused and way too fucking attached, says some bullshit about thinking it was an open relationship. The rest of the date he tries to bring back what he thought was a genuine acceptance of him as a potential mate ruined purely by him getting carried away – this is where we discover he has the emotional maturity and social abilities of a teenager, and can call him a creep for not moving on at that point. Up until then, he's kinda lame, but not doing anything wrong.

If you want to go out with friends, invite an extra. More people aren't going to hurt a hang out if it's really just between friends. I mean you can say she shouldn't have to invite extras, but you can not tell me two people opposite genders dinner and a movie doesn't sound exactly like a date. No way in hell is that not sending the biggest this is a date signal. If you just wanna be friends, that's fine. Just don't set up dates and acted surprised when people think they're dates. You can't play with people like that.