In both male and female demand, sex and looks are complements; all else equal, the better someone looks, the more you want sex with them. In male sex supply, sex and looks seem unrelated; how much sex a man offers has little to do with his looks. In female sex supply, however, it seems that sex and looks are substitutes; the better she looks the less sex she offers. Supporting data:
“Overweight or obese teenage girls are more likely than their recommended-weight peers to engage in certain types of risky sexual behavior but not others.” (more)
“Women on campuses where they comprise a higher proportion of the student body give more negative appraisals of campus men and relationships, go on fewer traditional dates, are less likely to have had a college boyfriend, and are more likely to be sexually active.” (more)
“Women in their 30s and early 40s are significantly more sexual than younger women. Women ages 27 through 45 report not only having more sexual fantasies (and more intense [ones]) than women ages 18 through 26 but also having more sex, period. And they are more willing than younger women to have casual sex, even one-night stands.” (more)
Apparently less-demanded women compensate by offering more sex, by requiring fewer “traditional dates,” and less insisting on official “boyfriend” status. They are, for example, more willing to be a second woman on the sly. Many don’t want to fully admit to making this tradeoff, however, and so would rather blame the men. (On the age effect, the study authors actually prefer to explain it as compensation for falling female fertility. But male fertility also falls, yet men don’t show this age effect.)
So why is the relation between sex and looks so different in female sex supply, relative to other gender and supply vs. demand combos? On obvious answer is that for women relations are primary and sex is more instrumental; women offer just as much sex as needed to get a man. For men, in contrast, sex seems primary while relations seem instrumental; men more enter into relations in order to get sex.
A related datum:
“Binge drinking significantly increases participation in sex, promiscuity, and the failure to use birth control.” (more)
Why do woman need to get drunk to have sex? It seems a way to stay in denial about intending to have sex – they can say they got drunk “for fun” and then the sex “just happened.” Are women in more denial than men about intending to have sex? That would make sense, if women were traditionally expected to exert more self-control on sex.
Added 5p: I should clarify that by “looks” I mean most any attractive feature, not just physical appearance.