Why would he do this?

So I’ve been talking with this guy for awhile now. I found him charming and funny, we’ve gone out once before and he said he’d like to again. We’re very flirty and he responds to my stories online... but a mutual friend of ours had a casual conversation with him and asked him if he had a girlfriend and he said yes.
I’m not sure why he would flirt with me and everything if he was seeing someone. It makes me feel kind of used and lead on. She’s apparently long distance according to my friend.
I haven’t asked him about it yet, I just don’t know why he’s so something like that. I’m not the kind of person to want to get in the middle of a relationship, nor am I a -the other woman- type. I just feel hurt.
Why would he do this?
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