The Fixed Point

This is actually a well-documented phenomenon, complete with hundreds of Yahoo! Answers queries (my favorite response: "well probly idk mayb there jus bored lol") and a book called, yup, Why Do Men Fall Asleep After Sex? Even Arianna Huffington has weighed in ("Men go to sleep because women don't turn into a pizza," Men's Health editor Dave Zinczenko informed her.)
There's no hard-and-fast consensus yet, and physical exertion probably plays a small-to-middling role in the post-sex snoozathon, but the chief culprit seems to be (as you guessed) the soup of hormones that men release after orgasm. Prominent among these is prolactin, which mediates a variety of physical responses, including sleepiness. Levels of the hormone are highest while we are sleeping, and mice deprived of prolactin are not able to get the amount of REM sleep that they should. Interestingly, prolactin also induces the sexual refractory period by suppressing the effects of dopamine, an arousal hormone. Actual intercourse produces about four times more prolactin than masturbation, which explains why men don't crash after a solo orgasm. (Check out the paper for more info on this interesting quirk of human sexuality.)
So there's a kernel of truth at the heart of this claim. But do men actually fall asleep after sex more often than women? Nobody knows; I certainly haven't been able to find any scientific evidence to back up such a strong assertion. The authors of WDMFAAS seem to think it has something to do with the extra work the male body does to produce an ejaculation. It's also possible that women get just as sleepy as men do after orgasm, but that they don't orgasm during sex as often as men do. Or maybe women fall asleep after sex just as often as men do, but men just don't notice or mind as much.
The verdict? Not precisely true, but not precisely bull.