I think WBS needs to watch Coupling(UK). They perfectly describe this situation. The gist of the conversation is that lots of girls promise a 3way later in the relationship, then once they marry him it drops down to a "maybe one day", from there he gets her preggers and she goes "hey honey, here's your 3way". It's a very funny episode that ends with one character getting a 3way with his gf - only she thinks he meant and another guy not another girl.