Not necessarily -- it could just as well imply that 19-29 year old women are having entirely-monogamous sex with men outside (hopefully above) the 18-29 range.
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]
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...