Hookups and "Counts As Sex"
Thank you for shedding a little clarity and rationality on the topic.
One of the things baffling me, though, is that I don't know what you (or the researchers) mean by "having sex". The contexts.org article, for instance, includes under "hooking up" activities that in my distant youth were called "making out", which was definitely *not* classified as "having sex".
Similarly, I have a lot of trouble with remembering who is supposed to be "receiving" oral sex, and wish people would just say "fellatio" when that's all they mean. It's extremely difficult, reading this material, to figure out who is actively trying to have an orgasm with whom and how often, which I figured was the Gold Standard of When It Counts, but maybe I'm just a fogey.
In "This Side of Paradise", doesn't Fitzgerald talk about a culture of "petting" and "petting parties" in the 1920s? -- which sounds a lot like hooking up to me, if hooking includes what us fogeys called "second base".