My co worker has a girlfriend but is flirting with me?

[Anonymous]

So ever since I starting working there he had always treated me differently. At first it was just compliments here and there, but nothing major. It seemed more friendly than anything. I always thought that what he was thinking was that he'd be into me if he didn't have a girlfriend. He'd mention her in past conversations. But these past two months he hasn't mentioned her at all and has been extra flirty. He's always saying things like "you're making me blush and look bad" or " I want your attention" in front of my other coworkers. I'm a cashier there and once one of my co worker was buying something and was talking to me, he came and told him to stop talking to his girlfriend and that he doesn't allow it. To another co worker he said that we were going to get married and things like that.
When it was his birthday he had asked for a kiss. I didn't think he was serious but then he asked me again a week ago. I didn't like that this was already getting this far so I told him that he had a girlfriend and he shouldn't be asking me that. He then agreed and said he shouldn't do a lot of things, and that it makes him a bad person.
I don't know what to think. I really like him and I don't think he's a bad guy but he does have a girlfriend, and he lives with her. What do you guys think? Does he genuinely like me or is he just bored?

Updates

+1 y

Oh and I'm 21, he's 22, and I believe the girlfriend is 27 to 29.

[HandsomeRaj]

1. He's probably looking for some extra excitement on the side maybe?
2. Nothing scarce in chemistry even if one sided developing suddenly
3. Ofcourse it's a compliment to you :)
4. I'd be more appreciative of him if he came clean
5. He's obviously attracted to you (now question remains if it's for sex or relationship)
6. I'm not even saying that if you and he got into a relationship and he's done this to her what stops him from doing it to you tomorrow :)
7. I think you need to let him go and if he's serious he'll come out in the open with whatever's on his mind. Till such time don't waste your time thinking about it :)