A female relative

"Well, since you're bisexual, I thought you might want to try sex with another girl. I feel like you should get to experience that at least once in your lifetime, otherwise you'd be missing out," you begin.

"And well, if we're really serious about this, every guy lusts after his girlfriend's hot sister. Or her mom, even. Your sisters are really good-looking. And you've said yourself your mom has barely aged a day since she had you," you point out. "Anyway, it would be really hot to see you with one of them," you finish.

You never would have dreamed you would say this out loud to your fiancee, but at the same time, you never dreamed she'd offer you sex with someone else. So you mentally cross your fingers and pray she will bite.