if your girlfriend cheated on you with another woman...

[tehweave]

It is cheating.
As 'awesome' as you think it is for two lesbians to get it on, (Now don't you deny it.) she still cheated.
So... Yeah.

 

[SimuLord]

In my relationships, there has generally been the option open for the girl to bring another girl into the relationship (I'm not against that sort of polyamory and have done it successfully in the past---my poly relationship lasted over two years before it broke apart over long-term considerations when one girl wanted me to herself as a husband.)

But if the girl just plain cheated on me, with no intentions of involving me? I'd break it off. Polyamory isn't promiscuity. It's a distinct type of relationship with its own rules and expectations which carries just as much burden of being respectful and caring on the part of all participants as a one-on-one.

 

[CarpathianMuffin]

I'd feel pretty betrayed, and certainly annoyed and heartbroken.

 

[ShadowsofHope]

It would still suck, and it be cheating, regardless of gender. I would feel pain, no less.

..But I won't lie, the thought of it would be HOT.

 

[Grumman]

That said, the male tendancy to assume that lesbian sex... [is] non-threatening compared to heterosexual sex is a bit fucked up.

It's quite simple: cheating with another guy is inherently insulting, because of the obvious question: "What does he have that I don't have?"

If she cheats with another woman, there is at least one non-insulting answer to that question: "Breasts and a vagina."

Sure, she's still cheating on you, but cheating with someone different is less of an insult than cheating with someone better.