According to The Atlantic, a study analyzing top ten Billboard hits from 2009 found that 92% of them are about sex (or, as the study calls them, "reproductive messages," which what? I think we can de-couple sex and reproduction nowadays, no?)
Those 174 top-selling songs were analyzed in order to determine how many sexy messages they contained in any of 18 sexy categories, including "arousal," "sexual prowess," and "genitalia." There was an average of 10.49 sex-related phrases per song, with R&B being head-and-shoulders(-and-maybe-some-other-body-parts) above the two other musical genres analyzed, country and pop. "Sexual appeal" was the most popular theme among both R&B and pop songs, while "commitment" (yawn) was most prevalent in country music.
Here is a chart breaking it down by music type and theme:
I am curious to hear the country songs that were about genitalia? Although I guess the real question is what the hell the other 8% of songs were about. Snacks?