The thing to remember, op, is that feelings and actions are two different things.
It is not always possible to decide your feelings - rarely if ever possible, actually, but it IS possible to decide your actions.
You make choices. This is what I was saying to you on your other thread. You didn't accept it then and I don't expect you will accept it now but you are in charge of your choices and you need to take responsibility for them.
What you feel is one thing, what you choose to do is quite another. Nobody outside of a crappy mills & boon novel is the helpless victim of their loins. In real life, people make choices.
You can decide to proceed and try your best to make this sack of shit choose you, abandon his partner (tbh, she'd have a lucky escape if he did. Sad that she didn't get it before she got pregnant because now she's got a lifelong tie to the bastard) and baby and shack up with you - then you'd be stuck with him and we can all pretty much know what life you'd have. But you wouldn't see it or accept it for another ten years.
Then you'd be older and wiser and it'd be too late because you'd be tied to him too, but you'd look back at 28 yr old you and you'd go why didn't I listen...
But that's your choice.
I feel very very sorry for his partner, because she is the innocent party in this and she is being crapped all over and she doesn't deserve it.
I'm sure you see this as some great romance, where you're star crossed lovers destined to be together. In shitty-chic-lit world, she'd be a shrew of a woman who never loved him despite him being the most honourable of men and eventually she'd run off with the milkman leaving you and him to raise the child and it'd be really romantic - difficult at first but you'd triumph against all odds and be a family...
In reality, he shagged you because you let him and then went back to his partner and baby and his real life and that's where he'll stay, sniffing around you when his partner is tired seeing to their infant child and not attentive enough to his poor little needs, to see if you'll lay down for him again.
He used your body because you presented it to him and he's a shit.
I really hope that you learn from it before it's too late.