Why do we wince at the idea of people over the age of, say, seventy having sex? Heavy people? Unattractive people?
In an early episode of Sex and the City, Carrie despairs after accidentally passing gas while in bed with Mr. Big. Humiliated, she scurries out of his apartment before he can dress to stop her. In a subsequent episode, the fabulous four friends discuss the significance of doing "number two" in a boyfriend's apartment: One had to feel quite confident about the relationship before doing this (alone) in the bathroom.
The body of course emits unpleasant smells on occasion. Add to this the relevant viscous fluids and it is not hard to understand why children, upon learning about sexual intercourse, often scrunch up their faces in disgust. Sex can revolt us. As good as it can be, it's still somewhat yucky. That's in part the reason that at the age of forty, Freud stopped having sex with the wife who had borne him six children. It was a little gross.
And that's just sex between a man and a woman. What about two women in bed together? or two men? The first scenario has recently become a popular frat boy fantasy. Whether that fantasy bears much resemblance to what lesbians in a committed relationship do together in bed is a question for which I do not have adequate space here. The action that two men can get up to, on the other hand, disgusts legions. Yale Law School professor William Eskridge concludes an exhaustive scholarly analysis of American opposition to gay marriage ("For Better or for Worse" is the name of the excellent book) with the insight that, at least until a year or so ago, vast numbers of Americans considered gay sex really yucky. And so disgusted Americans voted against gay marriage.