Am I his security blanket?

Just wondering after an 8 year relationship could I just be the "security blanket" or it was all for the sex? I fell in love with this guy but he has cheated on me multiple times. 4 months ago I called it quits because he had 2 other gf's. He's still dating 1 of them he cheated on me with. Well now he says he was wrong I am the one he is supposed to marry and have kids with. I saw him about 3 weeks ago for some legal issues over some property that had both our names on just removing his name so no more strings there. Made a mistake and we had sex so I know if he will cheat on her with me logically thinking he would do the same to me again. I just am wondering if anyone can explain his behavior to me I'm lost. He's still dating her, taking her on trips, family involved, having sex with her so what I am seeing is he doesn't have plans to leave her anytime soon although he says otherwise. I know the family they say he's in love with me and will grow up but he is 34 years old really how old does a man need to be to stop acting like a teenager? I just needed some advice from a guys point of view. I feel used and 8 years was a long time I feel like I may have wasted on him. Thanks if you want to know more just ask I just gave the short version because there is so much. As far as I have been I am not perfect but I have never cheated or even considered it even when as he puts it we were on a "break" meaning he picked a fight that morning then had sex with some random woman same day. I personally feel like he is a sexaholic and needs serious help for his own good but not sure what to think anymore. Thanks again.
Am I his security blanket?
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