I really dislike the euphemism "adult" for "sexual."
When I hear a phrase like "adult activities," I think of things like doing taxes and shopping for bathroom fixtures at Home Depot. "Adult entertainment" brings to mind midwestern grayhairs watching Lawrence Welk. "Adult material" could mean paper from old trees bound in leather from old cows.
Part of the problem is that a lot of mainstream porn strikes me as adolescent in style. My image of adult relationships is one of caring for a partner who's had a rough day, going for a romantic walk and reminiscing about youthful exploits, and so on. Depictions of such acts aren't very good for selling magazines, low budget films, and monthly website subscriptions. What I call "adolescent sexuality" is very genital focused, high in kinetic energy, and with a sense of the forbidden. "Girls Gone Wild" brings to mind copious alcohol in red plastic cups and a bunch of sorority girls ready to hump anything that moves. "Women Gone Wild" brings to mind a retreat in the woods with middle-aged women excited to hear guest speaker Clarissa Pinkola Estés.
I don't mean to imply that adult relationships lack sex. On the contrary, intercourse is something 35-year-olds, in general do, but 5-year-olds, in general, don't. But sex isn't the only thing that distinguishes children from adults and it certainly doesn't distinguish between 17-year-olds and 27-year-olds.
"Adult" and "mature" are often used synonymously. If your boss says "We're having an adult conversation," it's not an invitation to share the kinky exploits of your weekend with the conference call. It's a reminder that name calling, whining, and sulking don't have a place in a business meeting and that coworkers are expected to behave maturely with each other. Yet "M for Mature" video games like Grand Theft Auto feature hours upon hours of immature actions like stealing cars and killing hookers. The Seventh Seal is a movie with mature themes. Barely Legal Teens 37 is not.
Further confounding the terminology is the word "family" (which is taken to imply children are present). "Family-friendly content" is saccharine Disney films where a one-frame erection beneath a robe gets people riled up about indecency. But while adult doesn't imply sex (monks, nuns, etc.), it's an odd family (implying children) that doesn't start with sex.
Taboo subjects oft beget linguistic acrobatics. America may have no greater taboo than sex and no end of euphemisms with two backs.