Guy's fantasies - threesome's. Girl's opinion's welcome!!

Ok, so it's usually most guys fantasies to have a threesome with 2 girls, I get the whole 2 girls is better than one and watching them perform lesbian actions would be a massive turn on etc.

Here's the thing. Would you suggest it to your girlfriend? How many of you have actually done this (ask your girlfriend) and what is your view on bringing another person into the sexual side of your relationship - do you not think it could cause problems from your girlfriends perspective?

My boyfriend said it's a fantasy he'd like to fulfill 'in his life'. We had this discussion the other night (because i don't mind watching lesbian porn whilst he has sex with me), but because he knows I like that, he's thinking I should 'act out my fantasy'. Its all good and well watching it on the net/TV etc, but its another thing involving another person - something i dont want to do. He said that it's something he wants to do in his life with someone he cares about, i.e. his girlfriend and another girl - just getting off on watching me with another girl. I dont want to do it but he seems pretty adamant its something he wants to do. Hes pretty sure of the fact that there are girls out there (of relationship material) who'll be willing to have sex with another woman in the relationship - i think that would be a very very very small section of the female society to be honest with you.....

Don't knock it until you've tried it.
That is all.

I would do it if I wasn't in a relationship. I don't think I could do it with my boyfriend.