Ask the Resident (Sex) Advisor
The Resident Sex Advisor answers sex-related questions from college students
The way not to go about this is to mention how hot her friend Tracy is, and would Tracy be into a three-way? Talk to your girlfriend honestly about your fantasy to get a feeling for whether she'd be into it. Try to emphasize how hot you think she is, how accepting you are of her bisexuality, and how hot it would be to see her with another woman, going at each other lustfully, dildos-a-flying, so she doesn't get the impression you just want two women servicing you (even if that is what you want). If she indicates that she's game, try cruising for chicks together at a bar or a party. Let her pick the ones she thinks are attractive, and let her chat them up. In the event that you are lucky enough to score a three-way, don't hog the spotlight - sit back and let the girls have some time to themselves while you watch and jerk off, or double-team your girl with the other chick.
On the other hand, having a bisexual girlfriend can mean she will leave you for a woman. In fact, that's probably more likely than you realizing your two-girls-one-guy scenario. There are advantages and disadvantages to weigh.
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