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I second the question asked by Tom70: men, can you explain why it would bother you to find out about a threesome in your partner's past? I ask because, based on the men I have known, a threesome seems to be high on the list of male fantasies, so if past behavior is an indicator of future behavior, this knowledge about your girlfriend would mean you might have a chance of being involved in a threesome in the future.
My second question is, again to the men, why does it matter if it's a MMF or MFF threesome? From where I sit, it seems like the salient difference is that, for a heterosexual man, a MFF threesome would be all about his pleasure-- in seeing two women together and in being with two women. But, for a heterosexual male, a MMF threesome would be about the woman's pleasure-- I've never met a hetero guy who has talked about getting pleasure from the idea of a MMF threesome.
The whole bit about not being sure if you're the father of your kids if your partner was once involved in a MMF threesome is not relevant here. Obviously, if the threesome happened before you knew the woman, it would not be possible for any kids she had to be yours. And, if she's having threesomes with you, presumably you'd know that she was using protection. And, if she's having threesomes without you, behind your back, then that's called cheating, and it's a different issue.