And I'm pretty sure men and women differ on the optimal number of sex parters over the next five years...
And what would be the basis of it? They do NOT differ on the actual number of sex partners. So mens' perfect number would be let's say 10 partners, while womens' 5 they would admit to having + 5 they would rather not?
My optimal number of gallons of ice cream to eat over the next five years is ten gallons.
In the next five years, I will most likely eat more than ten gallons of ice cream. Why?
Got an answer? Now, map it to the present problem.
Also, one thing to note that can mess up the "male sexual partners must equal female sexual partners" is the occurence of rare extreme female outliers. That is, a small number of women with far more sexual partners than average (usually but not necessarily prostitutes) who are unlikely to be found by these survey takers.
I just did some reading about "Sociosexual Orientation Inventory", a simple 7-item test designed to measure one's openness to sex without love and long term commitment.
Here are the questions. How long will it take you to spot the huge problem ahead...
Score is: 5 x item1 + 1 x item2 (capped at 30) + 5 x item3 + 4 x item4 + 2 x (mean of item5, item6, and reversed item7)
Do you see the problem already?
Quite predictably, researchers report that men have much higher SOI scores than women in all countries. But the first three questions (ignoring non-1:1 gender ratios, differently biased sampling for different genders, different rates of homosexuality between genders, different behaviour of homosexuals of different genders, 30 partners cap on the second item, differently biased forecasts of the second item and other small details that won't affect the score much) - simply have to be identical for men and women, so the entire difference would have to be explained by items 4 to 7, which have relatively low weights!
The differences between men and women can be really extreme for some countries, Ukraine has 50.79±28.92 (mean±sd) for men, and 17.36±8.65 for women, which means that either Ukrainian men, or Ukrainian women, or both, are notoriously lying when asked about past and future sex partners. In most countries the differences are more moderate, with total 48-country sample's scores being 46.67±29.68 for men, and 27.34±19.55 for women. Latvia leads the way with smallest difference, and so most likely greatest honesty, with 49.42±23.61 for men, and 41.68±26.68 for women, what can be plausibly explained by just differences in attitude. (fake lie detector experiments have shown it's almost exclusively women who are lying when answering questions like that)
SOI seems to be considered quite useful by psychologists, it correlates with many nice things, not only other questionnaires, but country SOI averages correlate with various demographic, economic, and health scores in quite systematic way. Still, I cannot read papers about it without asking myself - why didn't they bother to perform this basic sanity check - which would detect huge number of outright lies in answers. And more importantly - what proportion of "serious science" suffers from problems like that?
References: The 48 country SOI study, fake lie detectors shows which gender lies more.