In the surveys I've seen before, the average number of reported female sexual partners per man did not equal the average number of reported male sexual partners per woman. Men overreport their number of sexual partners; women underreport. This effect could skew the aggregate reported results among the entire population one way or another, depending on other factors.
If the underlying trend is one of increasing inequality in male sexual activity -- fewer men having sex with more women, while women have the same number of sexual encounters overall -- then because fewer men have had sex, the proportion of all people who have never had sex would increase compared to previous generations, even if the average amount of sex across the board actually remained the same as in previous generations.