I'm kind of getting used to stories in which a couple of straight men have a threeway with a woman. At least the men are enjoying being with the woman, but it still seems a little sad to me that the guys aren't enjoying being with each other, as well. In those it makes more sense that both the men focus on the woman. But those stories trouble me less than the ones in which a previously gay couple essentially go heterosexual for a woman. The stories in which a woman has a menage with twin brothers really does kind of squick me out -- too incestuous. Again, I can see why that's a fantasy some women have, but it's definitely not one of mine.
It seems that in many of the stories, the people involved commit to the relationship being permanent. I think it's more likely that a couple would have a fling with a third person, then return to just being with each other, or add a third every so often. I don't know if it's kind of weird that I can suspend disbelief for stories with intergalactic space travel or vampires or werewolves, but want the details of the characters' relationship to ring true. But if the relationship aspects don't ring true for me, it has a real effect on my enjoyment of the story.