So much victim-blaming language going around here. "Hey, nobody would take pictures of her if she'd just stop wearing skirts!" Gee, maybe it's the douchebag taking the pictures that's the problem, huh?
Upskirt photos don't just "happen," they're taken intentionally by people who are willfully invading the personal space of another. There are numerous contexts in which one has an expectation of privacy. What is beneath one's skirt is one such context.
I suppose if someone uses a public bathroom and someone takes photos/videos of them from the adjacent stall, that'd be fine, too? After all, if you didn't want to be photographed, you wouldn't use a public restroom where there are gaps between the stall walls and the floor/ceiling.
People who are doing nothing wrong shouldn't have to adjust their behavior in order to thwart miscreants. The law should side against the miscreants.