Why would a guy keep casually mentioning that he has a gf?

intimidate wild female to stay away from his clutch of eggs!

Because he is really, really bad at flirting.

Is he also being flirty? I think he's trying to set up a casual NSA arrangement with you. He's letting you know that he's only so available

He’s assessing whether or not you would make a good third.

Stop with the crazy eyes, you're freaking him out

he probably thinks you're hitting on him.

...because he has a gf.

He’s not into you