How ridiculous is fidelity?! So let me get this straight, if I'm in a relationship, even if Milla Jovovich circa Return to the Blue Lagoon wanted to tickle my balls with her uvula, I'd have to turn it down? That seems a little ridiculous. As my friend put it, "C'mon. It's just like jerking off...but with someone else's mouth. I have a toilet at home, but that doesn't mean I can't take a shit outside the house! I just put paper on the seat first." Well said.
Jokes aside, if you can't handle fidelity, don't commit to a relationship in the first place. If you know you're gonna cheat, but you're too insecure to live outside of the confines of a "monogamous" relationship, you're just an asshole.
But let's examine this a little more closely. The measure of success from an evolutionary standpoint is the propagation of one's genes. This, in conjunction with men and women's very different biological compositions (i.e. - A man basically produces an endless supply of sperm for the duration of his life, whereas a woman is born with a finite number of eggs, the fertilization of which must occur within a prescribed window of time. This presents the genders with two very different optimal strategies of fulfilling the evolutionary mission of passing on their genes. Men should basically bang anyone and everyone in order to produce the most offspring, while women should be much more selective, breeding only with those who give their precious eggs the best chance of survival), often puts us in a difficult position given the societal expectation of monogamous union. I guess you could say it pits our entire evolutionary history against our relatively recently developed conscious brains. The fact that this conflict doesn't cause more problems is, I think, a testament to how far we've come in terms of brain development and impulse control (which, in turn, gives us the power to change the course of evolution, which, before us, was probably occurring only at a subconscious level).
But, having said all that, when you think about it, if evolution led to us, and we created marriage, then doesn't the institution of marriage, or at least the concept of monogamy, represent a gene-like unit that is, itself, the fittest because it has managed to survive thus far?
Anyway, point is, Milla, if you have a time machine and want to hang out some time...
(Special thanks to Roy Boy...)