In fairness, while you may be right, the OP has been extremely cagey about what actually happened after they’d met at work, and before her now-partner left his pregnant wife.
She says ‘nothing happened’ until after he’d left his wife and the baby had been born, but it’s not clear what that means, exactly. No sex? No kissing? No telling one another they were attracted? No explicit or implicit signals or promises that, if he were single, she’d be up for a relationship?
It’s actually an interesting question, given how often people say about affairs on here ‘Ok, you’ve fallen out of love with your spouse, and had your head turned by someone else, but at last have the decency to leave him or her before you start a new relationship’ — this, it seems, if we believe the OP, is exactly what this man did. He didn’t cheat, he ended his marriage while expecting a baby with his ex-wife, and only after they’d separated, started a new relationship.
Would this have been OK if not for the pregnancy? Given the pregnancy, should he have stayed with his wife for some extended period after the birth? How long? How culpable is the OP if she didn’t have sex with a married man, just told him that if he were single, she’d be interested? We’re perhaps more used to the narrative of the married man who tells his affair partner he’s going to leave and never does. This one did.
I say this in full sympathy for his ex-wife, who was left in an appalling position.