But the only reason you've had a lucky escape, is because it was somewhat forced, in the fact he wouldn't choose you? Had he left his DW and DC for you, then what??
I can understand the revenge feeling, I've been there but on the other side. And its just not worth it.
I do appreciate your honesty. I hope you never have to experience the pain you and the idiot you were sleeping with will cause to his gf and DC.
I can't say how I feel about you not telling his GF, I would desperately want to know if I'm being made a mug, among all the other things that I wrote already. But that is me, better to know, than not to know for myself personally. I think it becomes much more painful as more times goes by, and turns into years, then decade...etc.
Revenge won't make you feel good for long Hurry. You have to consider the gf, imagine your heartbreak then even more, they share a child together, she has no idea her partner is sleeping around, her heart will break itself for herself, her child, the trust, the images of you two shagging. You knew about her......and yet you chose to do this. :(
I still think she should know. But as someone else said, she won't thank you for it.