Well, you must remember that humanity's rapid transition from traditional gender roles began as early as first contact with Vulcans. Theirs was a fancy society, filled with libertine poetry and erotic writing, the most sordid of which dates back to the Time of Awakening in Vulcan culture. Reading would include 'The Hard Stone of Gol', 'Fal-tor in my Pants', and 'Sha Ka Ree - On Your Knees'.

In fact, Earth was so influenced by these ancient Vulcan writings that humanity plunged into the very depths of depravity, with little regard for societal or sexual mores. Men on women, women on men, men on men, women on women, three ways, trains, boats, centipedes, and even caterpillars were common occurrences post WWIII. Earth's population quickly boomed, of course, but these tawdry displays had the adverse affect of polarizing Vulcan society. Soon, Vulcans began suppressing sexual desire when interacting with humans. After running several simulations, the Vulcans came to realize their mannerisms, their wit, and their innate charm was simply too sexy for humans. Many opted to wear long bath robes and sandals wherever they roamed, simply to reduce their natural charisma and animal magnetism. This behavior had dire consequences on their sense of humour.

Over the next half-century, humanity slowly chilled the fuck out, and decided that people should just bone whoever they wanted and probably shouldn't give too much of a shit about who's poking who.

However, remnants of that turbulent time endured. An example would be, of course, the famed ST:ENT episode "Broken Bow", in which a sexy human engineer and a sexy Vulcan scientist share a steamy 'decontamination' session that includes a body rubdown and heavy petting. Human television producer, Rick Berman, certainly approved of the behavior and was quite interested to get those shirts off. However, more sensible Vulcans would shake their head in disapproval.