A quite simple but reasonable example might be tropic, Mediterranean or at least in any aspects hot climate. Consider amazons, for example: in such a culture, there's no problem with a lot of exposed skin.
In general, in these settings, it needs for comfort.
Also consider agility: certain clothings decrease the speed of moving legs. Of course, certain military units are not heavily relying on it, but if you want to create agile and quick units (ninja/assassin/scout archetypes are the most obvious IMHO), then you may gain advantage from it.
I mean, whatever you wear below the waist, that might be beneficial is it's tight, because it's harder to tear down, and might provide effective protection, compared to some loose dress. However, it also slows you down because it can constrict you, by obvious reason. Try running with huge steps in jeans.
These were my main considerations while exploring the topic; of course, society is also a defining factor in it. Certain cultures (such as the "general European culture and mentality", if I can define such) find exposed thighs and buttocks (especially buttocks) suggestive and strongly attractive. Okay, some may argue about it but at the very least, one's for sure in general: in said cultures, people are always paying attention to it, rather than finding it "natural".
If this pressure is present, then exposed bottom won't (or less likely will) be present, especially among war-related workers (=fighters/warrior/soldiers/etc)
This is why I started my answer with hot climate. Exposed skin is less of a "taboo" in these cultures. In ancient Egypt, for instance, women often left their breasts exposed - and it's still not uncommon in jungle tribal civilizations. It's, on the other hand, less likely e.g. in Europe.
TL;DR - to justify exposed bottom, three main considerations can be taken: climate, agility and society.
Also notice that two of these make it possible, if not naturally expected for men to have less clothing below the waist. Of course, if males and females are equal in this question, then the latter has no significant advantage from it.
In the end, however, it's kind of an instinct in males to be pleased by the view of female buttocks.
I'd suggest trying to explore this very simple fact, if you want significant advantages for females.