I'm thinking that the reason boils down to simple biology, or the biological imperative: In order for the human race to carry on, reproduction must take place. And reproduction requires penis to meet vagina, sperm to meet egg. This primal urge has determined human behaviors and proclivities from the earliest days.
Women can conceive without experiencing orgasm, but the same is not true for men. Thus, women are most gratified with intercourse; men, with ejaculation (orgasm). That's why the typical guy's sexual proclivities tend to be much more varied than women's: The most important thing to men, when it comes right down to it, is getting their rocks off. In fact, I've heard from more than once source that many men prefer fellatio to everything else, if they have any preference at all!
Now, don't get me wrong: I'm not saying that women don't enjoy many other sexual acts, or that men care only about orgasms and women don't care at all about them. Or, even that women desire only intercourse every time they make love. Tastes, interests and preferences can and do change, both in the short term and the long term. Sex drives can slow and mellow, or become more experimental and pronounced -- especially into middle age and beyond.
But, regardless of age, the biological imperative cannot be underestimated. And, here's another argument that bolsters my theory: If you think about it, almost all other sexual acts mimic the act of intercourse -- the motions, the sensations, etc. In other words, while men may not realize it or even think about it, male penises are really seeking female vaginas!
So, all I'm sayin' is: Guys, in order to keep your lady love satisfied, just make sure to give her a regular dose of good, old-fashioned screwing!
So, what's your reaction? Think I may be on to something? Think I've lost it? Let's discuss.