I don't necessarily recommend you read this post; there's a verbal picture or two from which some of you might run screaming.
What human beings find sexual or sexy is in general closely associated with youth, where that is understood as something like 'nubile youth'. But because what we associate with youth is heavily influenced by culture, the curious feature of this is that what we consider sexual or sexy is constantly changing, and almost necessarily so.
Take, for instance, the waltz. When young people first began dancing the waltz, it was a considerable scandal throughout Europe. And it certainly was young people; only they would have been risque. For prior to the waltz, dances involving men and women typically would either involve no touching at all or touching only at the hands; also, there was clearly demarcated personal space among the dancers. The waltz broke both of these rules of propriety. In a waltz the dancers are face to face and chest to chest. And, what is worse, the man's hand is squarely on the woman's waist. Close in, with the man's hand on the woman, moving to a beat -- ONE, two, three; ONE, two, three; ONE, two, three; sway, rise, fall; sway, rise, fall. It was virtually sex on the dancefloor. And if you think about it, the old guard who thought this were quite right; the waltz is very sexy in these ways. And in some ways it is more like sex than most of the more vulgar dances one can see in nightclubs these days. Better tempo, for one thing, and you can look right into the eyes of the person you feel right up against you.
But, of course, the young men and women who danced the waltz when young continued to dance the waltz as they grew older. Dancing instructors tamed the waltz by establishing a clear frame and rules of propriety for those who were squeamish. And eventually the young were no longer young, and still dancing the waltz, and if you went to a dance you could see your grandparents dancing the waltz. And just try to think simultaneously of the waltz as sex on the dancefloor and of your grandparents dancing the waltz, I dare you. If your grandparents dance the waltz in public, you are going to have a little bit of a problem thinking of it as sexy, aren't you?
One wonders at the limits of this. Mercifully, I think that when I am in my eighties I will be spared the sight of my fellow senior citizens dancing the grind, for the safety of their hips, if nothing else; but, of course, it could be that by the time I am in my eighties dancing instructors will have begun teaching tamed versions of the grind for those who don't like that much touching, and everyone will regard the grind as the sort of thing old people do at weddings. Nobody will think of it as risqué.
Or it might break off as a sort of subculture. Polka did that. It's common to think of polka as very silly and old-fashioned music for very silly and old-fashioned dancing; but obviously nobody thought Polka was old-fashioned when it first came out, and nobody thought it silly, either. Polka is dance music, and like all new dance music it was associated with sex-obsessed young people -- young men showing off for young women, to get them interested enough to go farther than dancing. That's why polka has so much beat to it; and why the bare beat, if you consider it alone, starts sounding a lot like the dance music that is played in nightclubs today. We think it's silly because of the tubas; but the thing about tubas is that they are good for conveying a LOUD BASS BEAT if you don't have amplifiers. You don't just hear a tuba, you feel it, just as you feel the drum track blasted in nightclubs. It's dance music, differing only in the preferred instruments. Perhaps the sort of thing that is done in nightclubs today is the sort of thing that will eventually be seen the way we see polka. Perhaps it will be a thing we associate with odd European festivals, in which all the men wear jeans-and-boxer combos rather than lederhosen. It won't seem sexy; it will just seem weird (or quirkily fun, if you like that sort of thing). Oompa-oompa-oompa. Or, rather, uhntiss-uhntiss-uhntiss.
Or perhaps we really will be subject to the sight of grandparents and fathers and daughters grinding, God help us. What we can be sure of is that what people think of as sexy will have changed radically. The young will find new things to push the bounds of propriety because what their elders saw as pushing the bounds of propriety is something the young think of as associated with their elders. For my own part, I root for the least probable of all the scenarios, in which the young again dance the waltz on that line between what you can and can't do in public. If nothing else, the music showed more genius then.