So he has few morals (she's not only married she has kids - how old are they? Are you sure none of them his?) m
Sounds like HE was an unfaithful husband too
"Its a long complicated story about him cheating on his wife, it was over with anyway and they were in the process of divorce but still living together when it happened" of COURSE "it's complicated" 🙄 I take it this is just what HE told you? Ever spoken to his ex wife? Seen their divorce papers?
"They don't have children together as she was married before they even met." That doesn't mean none of her children are his!
Keeps bouncing back to her - "just sex" nah not buying that
Still in contact with her
LIED to you about her
Personally with all you've said - I think it's distinctly possible you're a type of beard - so she can say to her husband "of course I'm not seeing him any more he's got a new woman"
NO you don't get to call her (or any woman actually) a slut! She hasn't behaved ANY worse than he has!
Get rid of him - he's REALLY not that into you and your DC deserve MUCH better too (personally forgetting DC's birthday I'd bin for)
Get therapy - to address your low self esteem, low standards in relationships and poor view of women but overly forgiving view of men