I'm only 21 but have already overcome this mental blocking way of thinking.

Its cool if a girls been in a threesome! It is actually beneficial as she will not feel the urge to cheat on you with two strangers to fulfil the desire to have one in the relationship.. plus if shes game for a threesome, get her hot chick friend in!

And as for exs in social groups.. your the one banging her now, not him!