You get a guy in a room and you sit him down in front of a television. Let him strap a Rigiscan to his penis (exactly what it sounds like) and give him a dial he can tune up and down (“I feel a little aroused, I feel a lot aroused” etc), put a tray over his lap, and then show him some porn.
With about 85% accuracy, the tumescence of his penis will match the level of arousal he dials in. It’s not a perfect correlation, but it’s really, really high. The more erect he is, the more aroused he is – or, as porn star extraordinaire Nina Hartley puts it, “Is his dick hard? Then he’s liking it.”
Now let’s do the same thing with a woman. Put her in that room and that chair, and let her insert a vaginal photoplethysmograph (a little flashlight, essentially, that measures changes in bloodflow), and give her the dial and the tray and the porn.
There will be, gosh, just no clear relationship between what her genitals are doing and what she dials in as her arousal. Even orgasm might be nonconcordant – a woman’s physiology might say “orgasm” on the machines, but she may still report not having an orgasm. Ditto the reverse.
Further, women’s genitals will respond to, like, ANYTHING remotely sexually relevant (lesbian sex, gay sex, het sex, monkeys having sex…), while men’s genitals respond pretty exclusively to the thing that they’re oriented toward – women or men having sex with men, depending. For women, genital response IS NOT A MEASURE OF DESIRE. Just because she’s wet and/or swollen doesn’t mean she’s interested, it just means her body has prepared itself for a potential sexual situation.
Why? Christ knows, really. It seems likely that women’s bodies respond to a wide array of sexually relevant stimuli because penetrative sex is high risk for females – we have double the risk STD infection from penile-vaginal intercourse compared to males, and lubrication etc helps prevent tissue damage that can facilitate infection.
So that’s nonconcordance.
The important thing to remember is that it’s not what her body’s doing, it’s what her mind says that matters. Her body is a muddy, vague, organic heap that just wants to prevent infection and unwanted pregnancy and other not-so-good consequences. Her mind understands the social world and has opinions that go beyond biology.