OP I get exactly what you're saying, despite the sanctimonious only being able to see as far as their own nose.
You wanted something as a single woman, fun, hassle free. You met a man who led you to believe he was in the same situation.
Because he's a liar, you've been having sex with someone's partner. With three children involved. You've been unwittingly allowing someone to "cheat" with you. I absolutely get why you're furious. I would never engage with someone who had a partner, for a multitude of reasons, but mostly moral. To find out I had been tricked into doing something so intimate, that I would absolutely not have done if I knew the truth, would enrage me too. I would also feel guilty, regarding her. Because now you're aware you've been part of his cheating. You're feeling all the shitty things that a cheat feels, (or should feel, most probably don't.)
You don't know what he's told her. The fact he's on there already again, this fast, means he's a c*nt who has no respect for her. You've shown her your evidence already. You've done enough, it's her choice what she does with that information. You can lead a horse to water and all that...
Forget him. And her. But allow yourself to get over this feeling of being disgusted in your own time. It's a valid feeling. What can you do to make yourself feel better instead? Out for a nice meal, spur of the moment tonight? Book a haircut? Is there something you can treat yourself to to signify the close of this horrible chapter and moving forward positively? 