It's the classic sex act every woman hopes she's never asked to perform."Tonight, I'd like to pretend we're strangers—you'll be wearing a gigantic raccoon costume answering to the name 'Mayuka,' and I'll be in a studded leather diaper—we'll meet at a boarding school for kid wizards and spank each other with stiletto heels while Teletubbies plays in the background. And let's film it!"

OK, maybe your partner has never approached you with a bedroom request quite that elaborate… at least, one would hope he hasn't. But chances are, at some point he'll reveal a desire to you that at best catches you off guard, and at worst makes you wonder whether you should alert the authorities. When that happens, the way you react could have ramifications not only on your sex life, but on your relationship as a whole.

"Negotiating about one's sexual repertoire in an honest, and respectful manner is a necessity for any couple," says Erika Pluhar, PhD, an Atlanta couples and sex therapist and adjunct assistant professor at Emory University. "A woman can respond that she is uncomfortable with a particular idea, or she may decide that she could become comfortable with the behavior. But she should be careful that her response doesn't shame her partner for asking."

In other words, no face-making, pretend vomiting, or merciless taunting. Sharing sexual desires, especially those that may carry a stigma, is an intensely personal act that may leave your partner feeling vulnerable. Even so, you should never do anything that makes you uncomfortable. But rejecting the act without seeming as though you're also rejecting your partner can be more difficult than it seems.

One way to handle the situation is to deal with your partner the way you'd deal with a petulant three-year-old. That's not to say you should be condescending, or scold him, or spank him… unless that's what he was asking for in the first place. Instead, tell him you'll have to think about it, then shift the focus by asking him questions—what sparked his interest in this fantasy? How long has he been fantasizing about it? How does he see it playing out? Doing so may help you understand and accept his desire, and it'll also help him understand that while you may not want to do what he wants, you don't judge him for it, either.

"It's OK to tell him that the sloppy way he loads the dishwasher makes you angry, but with sex, the talk should always be positive," explains Alex Caroline Robboy, a Philadelphia sex therapist and founder of Howtohavegoodsex.com. "Your partner is taking a risk, and you don't want to slam the door so hard that he stops taking risks."

In fact, unless what he's proposing is against the law in New England and the Far East, you should always leave that door open just a crack—never say never. Not only is it incredibly negative, it also eliminates the possibility of compromise or change.