Also, that 18-29 chart is about people looking for a relationship. The data about actually-having-sex [1] used different age ranges, breaking up 18-24 and 25-34. It shows a spike of no-sex in 18-24 men, which evens out with women once they're into the 25-35 range. The article didn't use the handy graph in figure 1 from the study which was quite informative: [2]
[1]: https://jamanetwork.com/journals/jamanetworkopen/fullarticle...
[2]: https://cdn.jamanetwork.com/ama/content_public/journal/jaman...
My hypothesis is that this is because of a pretty significant gap in EQ and maturity between young men and women. Also women’s prime reproductive years (physically) are from late teens to early 30s. Men’s prime years to be able to support their partners is usually starting around 30s. Makes sense when you think of it like that.
Based purely on that figure, we can reasonably say that after age 18-24 men and women are about as likely to have had sex with at least one person in the last year (within the error bars of each other), with men being more likely to have had sex with more than one person in the last year.