He supposedly has a girlfriend, so why is he acting like this? How do I stop it?

I met a guy at the gym that I've become pretty good friends with. I found out he had a girlfriend, which is fine. But, he's always mentioning checking out cute girls and have asked me to introduce him to my cute friends. He was telling me that one of my older friends is really attractive. I told him she is unavailable, and he made a comment that maybe he should wait for them to break up. I was a little surprised by his attitude, considering he has a girlfriend. He hasn't told me very much about his girlfriend. The only time he talks about her is if i bring her up. I haven't seen any pictures of the two of them together on fb, nor are they official on fb either. I know they met online, and they've been together for only a few months. He said she's planning to go back to school. He's always making comments about her weight. He's "supposedly" in a relationship, but still acts like he's single. Now, I'm starting to doubt if he even has a girlfriend.

A few days ago, he messaged me asking me to help him get to know a girl that caught his eye at the gym. I told him I didn't know her, but he still wants my help to get to know her. I called him out on his behavior by reminding him that he has a girlfriend and he shouldn't be trying to befriend cute girls. He insists that it would just be innocent but I don't believe him. I told him that he shouldn't be doing that if he's in a committed relationship, but he thinks it's perfectly fine. He said "just because im in a relationship doesn't mean other girls become unattractive. I just want to become friends with her and get to know her at the gym." I told him that I think he has more than friendship on his mind, and he swears that it's innocent. As far as I'm concerned, he shouldn't be looking at girls at all, since he already has a girlfriend. I think that's messed up. I don't plan on helping him, but he insists on me helping him to break the ice between the two of them. I called him a wimp.

Why does he keep doing this? How do I get him to stop?
He supposedly has a girlfriend, so why is he acting like this? How do I stop it?
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