AIBU to want to tell the gf

[Sunrise234]

I met a guy a few years ago and we had amazing chemistry. At first it was a bit of fun but it developed into something more serious as time went on. We never lived together but spent most days/nights together so I was shocked to find out from his friend that he had a gf who was pregnant! Obviously I ended things but he kept saying how they weren't together etc, a couple of years later after messaging me everyday begging for forgiveness and promising they weren't in a relationship I finally caved and we started things back up again. I went to his work do's, family parties etc had no reason to believe him and his DDs mum was still together. I took it slow again and a couple of years later again I found out the same gf was pregnant with his child!
Basically he is and always has been in a full-blown relationship with this woman and now has a daughter and son with her. Both times she was pregnant/just had a baby so I decided to not say anything to her as many other people knew about me and him and could have told her but didn't and I didn't want to mess up a family.
I was obviously hurt and upset but I realised that I had dodged a massive bullet.

I have recently found out that this guy has been sleeping around with everyone and anyone including friends of the gf and complete strangers and often visits sex clubs and is on various sex websites. I don't agree with cheating on any level but having sex with random women, sometimes groups of them and then going home to have sex unprotected with his gf makes me feel a bit sick. I also think about how embarrassing it must be that everyone knows about these things apart from her.

AIBU to want to tell the gf? Just so she knows what's going on and can protect herself.

If I were her, then yes, i would want to know. But many on here will suggest you keep your nose out.
Who's to say also that she doesn't know already but chooses to turn a blind eye? Nothing is ever simple in life.
But it i were you, i would give her enough information so as to prove your word and then move on and say nothing to either of them ever again.

Of course she knows!

His family know, his friends know, he's shagging her friends and you think she doesn't know this?

If he's taken you to family parties I'd assume that she knows he's not faithful and turns a blind eye.

Yes, she knows. I feel so sorry for you both. He’s a complete idiot.

Surely she knows and for whatever reason ignors it

I would think she knows on some level too as surely she wonders where he goes all the time and things. When I go to family parties she is not there but all his family and friends are so it would be weird that no one has said anything to her.