Men and women both really like sex, but there are some
The great majority of men want it any time, at every single moment, badly, and with anyone
they can get it with. This is simply not true of any but a very few women.
This is not to say that women don't love sex; they do (as much as men do), and they can
see a great looking guy and want to rip his clothes off then and there. But not any
guy, and they don't think this every moment of their waking lives as men do. (This,
by the way, explains the very different sexual behavior of gay men and gay women).
Guys do, of course, say "no" to sex from time to time, but that is for reasons
of a different kind; they'll often still want the sex, but they'll want even more to avoid
some social or personal ramification that for whatever reason would go along with the sex.
Again, women do this too, but guys will say "no" in this way much less
often than will women.
Another key difference is the sequence of caring and sexual desire. For men, they
will often begin caring about a woman after they've slept with her, and almost always
after they've been strongly sexually attracted to her. For women, this process is
very often reversed; it's because they've come to care for a guy that they begin to find
him sexually attractive.