i've did it. my ex-girlfriend made a deal with me; if i'd consent to having a three-way with another guy, she'd also do it with another girl. i got suitably drunk, and it happened. it was alright, really. actually, it was better than two girls at once, but that may have had something to do with the other chick totally smelling like clove cigarettes. yuck.
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They're all human beings, and though she may be a liar and a manipulator, it's probably because she doesn't know any other way to survive.