2 + 1 = ?

"Excuse me, ladies... would you like a hot dog for those buns?"

"What's up with men and girl-on-girl, or wanting to bring another girl into the bedroom?"

Variety, is what.  Keeping in mind what I've said about "new shoes," men desire something different than... well, you every night.  Sometimes this takes the form of another woman.

Within relationships, pressing for a threesome can be a man's way of testing your commitment to satisfying him sexually.  Women talk a lot about spicing things up sexually with their men, but lackluster stripping in the living room and unflattering lingerie (if you're not hot naked you won't get much hotter with a teddy on) don't really get it.  If you're willing to explore your bi-curious side-- even against your own inhibitions-- then he'll see it as a sign that anything goes.

Girl-on-girl provides visual creatures such as men with the equivalent of a double-scoop ice cream cone placed in a child's hands; the kid might not know where to start-- or how he'll finish-- but he's down to try, dammit!  To be sure, some men would rather watch two women having sex than to actually participate.

"If I agree to join in a threesome, how will this affect the way my man sees me?"

If he never valued you in the first place, it'll make him view you as just a powered screwdriver for which he's acquired another interchangeable bit.  If he does value you then you shouldn't worry about whether this knocks you out of the running for wife status.  This might actually put him more at ease with spending the rest of his life with you (again, if he truly values you).

If your man brings up or proposes a MFF scenario, ask him to explain why it's so important to him that he's willing to expose your relationship to third-party interference.  If he can't muster more than a six-word argument-- "'Cause that shit's freaky as hell!"-- tell him to work off that extra energy at the gym while he listens to Usher's "Little Freak."