Valentine's Day Massacre: Ex's

Would someone please explain the reason that some ex's just cant be gotten rid of? I have one who I have dated off and on for 2 years now. He's dumped me, I've dumped him, and once again he's coming back for more and I'm once again listening to him. I know good and well this relationship is going nowhere. We will never be together in a stable relationship. He is pathologically incapable of fidelity. I would trust him as far as I could throw him. And yet, he's calling again, wanting to see me again even though he wants to get back together with his ex wife. This is entirely sexual, I got over the love part months ago. I'm not the first person to notice the sort of behavior in humans. I've seen plenty of friends, some from different cultures, be off again on again with the same person for YEARS with no good results. One pair of friends actually married, divorced, remarried, then divorced again and just live together now. With their kids. Are we as humans so damn afraid of change that we put up with the same crap for the sake of safety? The multi-married couple say, "Well this person is a nightmare, but they are my nightmare." Huh?
This ex of mine is up for a dire surprise. I have other interests now, new friends and lovers. He's not getting the amount of attention he once got from me, and it is why he's pushing so hard. Sorry Cochise, not this time....