Jesus, what a pack of homophobes and hypocrites. You+your girl+another chick=awesome; you+your girl+another guy=that fucking whore has been cheating on you.
Have your best friend over and pull a Chinese Finger Trap on your girl. You'll love it, she'll love it, your friend will mow your grass for free for a year, everybody wins. Or split the difference between the fantasies and swing with another couple (might be easier on both of you anyway, since sudden onset of *ick* won't totally screw up the moment.)
Phaedrus