Well said. This is not complicated. Does it make sense for this character, in this scene, to be dressed sexy? If yes, then the character should be dressed sexy. If not, then not. A female fighter in the midst of battle should not have cleavage spilling out of her breastplate. The same fighter attending a ball at the royal court? Sure, portray her in a low-cut gown.Have you read what Mearls has said about it? He has said that creatures that are sexual (think Succubus) will be scantily clad as it fits their function. Why would a female warrior wear armor that has her boobs popping out and most of her chest uncovered? What kind of protection is that? They are trying to make the art more realistic. Also you point about people in deserts not walking around fully clothed is funny as most desert dwellers are covered head to toe as it protects them from the sun. I believe what Mearls is going for is a realistic portrayal of the characters/monsters, so they will be scantily clad when it fits but if it doesn't fit then they will be dressed more realistically.
I personally would even be okay with having the succubus portrayed stark naked*; but keep in mind that what's good for the goose is good for the, ah, gander. If you want R-rated succubi, then you better be prepared to flip a page in the Monster Manual and see full frontal incubus. If you don't want to see that, then maybe the sex demons ought to keep a few clothes on.
[size=-2]*I do not pretend to speak for everybody when I say this. If I were a parent contemplating introducing my kids to the game, I would probably feel differently.[/size]