That citation is from 1987, so I doubt it's accurate anymore.
Which one, the friends to lovers being the most popular one, or the one in the paper about 50% of romances having rape in them? I'm assuming the latter.
I read fantasy/short piece years ago in Playgirl of all places (we used to have read-alouds in my college dorm), where the gal was on a subway going through a tunnel, and the lights went out, and the guy behind her got, um, frisky and they, well, you know whatted, while standing, in the anonymity of darkness.
When the lights came back on, she "discovered" that it was really her husband, who just so happened to be on the same train (unbeknownst to her up until then).
Unexpected sex (often with a stranger or someone who is known but one has not had sex with before) is a really common fantasy for both genders, and possibly for the real biological reason of not "wanting" to put all your reproductive eggs or sperm in one basket (that human sex is most often social, not procreative, in nature is another factor that may be important here too). So is unplanned sex in "forbidden" places, even in places where discovery could be disastrous. Maybe the desire to prove oneself to a partner by taking sexual risks plays into some ancient test of fitness place in our brains too.
It's all speculation, though, and impossible to prove.
The thing is, we all have fantasies we know we don't really want to live (and that we couldn't live and that we know shouldn't be lived) in real life. I read a story once where the whole premise was something that would be completely illegal "quid pro quo" sexual harassment between the male boss and a male job applicant during a job interview. It was still hot.
But such a scene in a work of non erotic literature would not be hot and if it's portrayed as sexy outside of the context of a carefully constructed erotic fantasy setting, then it would feel wrong in the same way that a scene in a mainstream fantasy novel where two people have sex while fending off sword thrusts during a battle would.