It's not, I hope, because that way they get to LOOK AT NEKKID PEOPLE ZOMG. It's because they think the cultural taboos on nudity are silly. Who decided what body parts we must cover up? Why are bare buttocks obscene, and bare feet are just feet? Why do some cultures think it's immodest to expose one's hair? Are certain parts sexual only because we've fetishized them to be so? Do we like it better because we have?
If these parts were not taboo, would it be taboo to touch them? To exist in a polite society we have a standard that people are generally not touched without their consent. But we seek that consent often. We extend our hands for others to take them and shake them. We open our arms and lean in for a hug. During meaningful conversation, we seek to deepen communication by reaching to touch one's arm, one's shoulder. We are a society that does touch and considers it acceptable to do so as long as the other person sees it coming, does not withdraw or speak against it, and as long as the touch is on a body part not considered taboo.
So what if we took the taboo off body parts?
If I were out in public with you and you were someone I knew and trusted and you ran your fingers through my hair, I would be in touch-related ecstasy. I would sigh, "Ooh, do that some more," and if you continued, no one would run over and arrest us. They might think we're odd, but because you are not touching a body part considered naughty or dirty, we could have a field day. Fondle my breast or buttock or genitals in public, however, even if they're covered with clothing, and there will be cries of, "Stop that! There are children here! You can't do that in public! Etc.!" Why is that obscene, but the other is not?
If I proposed that we do a social experiment where we treated the public touching of sexual body parts with the same politeness we do non-sexual body parts, would people think that was interesting? Refreshing, liberating? They might. Could there be disapproval? What about people who did not want to participate? Would they fear I might label them as "hopelessly stuck in society's arbitrary rules," and be unhappy at such negative labelling? Might people think I just wanted to get my hands on their naughty bits?
They might. I would be hesitant to propose such an experiment.