Help my friend with a girlfriend is flirting with me?

Anonymous
Okay so, my friend has a girlfriend. He says that he's going to marry her when he's older, but he keeps on flirting with me. He told me that I'm cute, hot, gorgeous, and he even said that he would bang me. And I told him "you have a girlfriend" and he said "Sorry but you're so hot". He even download the pictures of my face, and he keeps on saying flirty things.
Like he said that the more he looks at me the hotter I get. And we had a conversation about my parents, then I said that I once walked in on them doing, you know what and he said "can we be like your parents" and he laughed after that.
He also made fun of my height, calling me cute and so on.
The thing is, I like him too kind of. Our personalities are very compatible, but he has a girlfriend. And I try to imagine being in her situation. If I was her I wouldn't be happy if my boyfriend flirted with other girls, calling them cute, hot, and so on. And even if I did get into a relationship he would probably cheat on me as well since he looks like to want to cheat on his girlfriend.
By the way the girlfriend doesn't know about his behavior.
I can't cut him off either, I've been friends with him for a long time and he has opened up to me about so much. I've opened up to him too. He told me about the time he got r4ped and it was really hard for him. I was there for him so I feel like shit if I would leave him.
But I don't want to be his side chick either.
What do I do?
Help my friend with a girlfriend is flirting with me?
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