[forgottenpass]

I can't think of any particular reason other than that it would be "weird" because we're not used to it.

Cultural standards are products of an evolutionary process. They can seem arbitrary and contradictory because, well, they are once the evolutionary purpose is gone. The american taboos around sex are largely driven by religion. Just look at the current fights over gay marriage. As more people lose their religion (or become "just christmas" Christians), the drivers of those standards will be loosen up, but that doesn't make the taboo go away, just introduce the possibility.