I've been seeing someone for a few weeks and by pure accident I discovered this morning that he has a partner and a new baby.

I'm am absolutely raging angry. I would NEVER knowingly sleep with someone in a relationship and I hate that I've basically been the other woman while his poor girlfriend looks after their baby.

I am so tempted to contact this girl and tell her. A large part of me believes she has a right to know what kind of **** he is but they've just had a baby. I don't know what to do.

What would you do?