For the sake of discussion, I'll agree with you that it's a matter of culture, not morality. That doesn't make it right. I'll make a comparison. There are many places in the Middle East where it is offensive to show the bottom of your foot/shoe to another person. People will not sit in a chair so that their leg is perpendicular to the ground, and thereby exposing the bottom of the foot to another person.
Now you or I might think that's silly. There's nothing immoral about it, and you're not doing it to offend anyone. But that doesn't matter. If you don't know the majority standards of decency, and show your foot, that's forgivable. But if you do know, and do it anyway, you are committing an offense against decency, IMO.
And a note regarding "activism": a person may say to themselves "the local custom about feet is stupid and irrational, and these people shouldn't think that, so I'm going to show my foot, and I'm going on crusade to convince these people that they're wrong and I'm right." This type of activism, IMO, is completely arrogant and wrong. The majority consensus of a culture is determined by democracy, otherwise it wouldn't be the majority consensus. To attack it is being elitist and saying that you know better than everyone else, and the majority needs to conform to your will.