I usually lurk here, but finally joined to tell you, OP, that:
First of all, I have been where you are and I know what it is like to believe on some sort of cellular level that you have found The One. In my case, he was definitely NOT the one. There is no The One. There are only men who WANT to treat you as you deserve to be treated. But somehow at the time I was not able to imagine ever moving on. The things he'd said. The way he looked at me. I honestly feltand he vocally concurredthat we were the only two people in the world who had ever connected on that level.
Well, until his wife called me up on the phone one day.
And I STILL thought I was his The One, and that she was a temporary obstacle to him finding me again and giving up everything to be with me.
Hormones and love chemicals can be a real bitch.
But years after all that, I witnessed just how badly people can be manipulated by lovers who lay on the "you're special you're the only one we were made for each other" charm.
I had two close friends who were involved with the same man in our friend circle. Everyone knew he was a womanizer, but these two women both had it BAD for him and were convinced of a deep connection, etc. This went on for a couple of years.
It went on until one of the women killed herself. She killed herself over this absolute bastard. That was when I knew that we are all so vulnerable. She wasn't an unstable person. I know how easy it is to assume that she was if you didn't know anything else about her other than what I've shared here, but she just wasn't.
I'm not saying you're suicidal. I'm saying our hormones and the sweet-talking of someone we're sleeping with can TOTALLY cloud our thinking to terrible extremes.
For the record, I don't believe the men do it for sex. I think they do it because they desperately crave romance and excitement. I think they do it for the same reasons women lead men on horribly. Because they need to feel wanted and because they are enamoured with themselves and their image of themselves as irreplaceable lovers.