At what point is it crossing boundaries with flirting?

A coworker has a girlfriend but he flirts with me quite a bit and I’m just wondering at what point is it disrespectful to her or too much?

He shows signs of general attraction- He checks me out a lot, leans in really close in my personal space, flirty banter/teasing, gives locked eye contact, all on a regular basis. Is this normal or is he crossing the line?

I did like him until I found out he had a girlfriend, but now I feel uncomfortable and a bit annoyed that he continues to do this, when I wonder why he bothers,

it feels it’s a bit like teasing, I like him, but try not too because of his girlfriend, so why doesn’t he just quit it?

I kind of dread seeing him most days.

At what point is it crossing boundaries with flirting?
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