You are right—age could be very important—and I didn’t think of it. For example, if men preferred much, much older women, most women in the sample would then have few sexual partners. Both men and women would, by the time they died, have had the same number of (hetero) partnerships on average, but whilst they were alive and around to fill in surveys, the numbers could be quite different.
However, the effect is probably in the wrong direction to help explain the reported difference, women live longer, and men seem to prefer (somewhat) younger women.
You are right—age could be very important—and I didn’t think of it. For example, if men preferred much, much older women, most women in the sample would then have few sexual partners. Both men and women would, by the time they died, have had the same number of (hetero) partnerships on average, but whilst they were alive and around to fill in surveys, the numbers could be quite different.
However, the effect is probably in the wrong direction to help explain the reported difference, women live longer, and men seem to prefer (somewhat) younger women.