It seems to me that men often want a woman with less than them, because they don't like the idea of her having more sexual experience than him.

I agree. I also find it to be true that women don't generally like men to have less sexual experience than they do, but I feel like the correlation isn't as strong.

So, why? I think it all goes back to cultural defitions of masculinity, ergo: gender norms. A lot of masculinity is tied up in being sex-driven and consequently being very skilled at sex. A man with a high partner count is a man's man, who was able to woo a wide array of women, and he presumably knows what he's doing sexually. A woman with a higher partner count arouses a fear in some men that, a) she will recognize that he's not "good" at sex, or b) he won't be as "good" as her past partners. I feel like this insecurity--that being with a more experienced woman will show the guy up as less "good" at sex and therefore make him feel like less of a man--is behind this. And, to some extent, women buy into the same ideas about men with more or less sexual experience being more or less manly.

Of course, cultural notions of femininity including women being more passively interested in sex likely make some men see more sexually active women as less feminine, so I know it isn't entirely about insecurity, either.

Edited for clarity