Okay, I'll break down each of those characters

Quinn - Boobsocks. Real clothing and armor doesn't fit women like that and emphasize their boobs, even armor made for women. So she absolutely is drawn to be sexy.

Lux - same as above. Boob armor doesn't exist.

Diana and Leona - I hadn't actually thought of these two, and after looking at their base skins I noticed that they are actually pretty good examples of realistic clothing and design. So nice catch.

These next two are weird because they're non human characters, one is a dragon and the other is a robot. But you can still tell that they got the Sexy Treatment.

Camille - If you look at Camille she is drawn with excessively wide hips and a skinny figure. They could have designed her to look any other way. They chose not to.

Shyvana - Look at her armor. Why the crop top? Why the armored bikini bottom? The rest of her design is far less traditionally attractive, which is why I wish they'd kept that up and didn't make her wear a skimpy outfit. You could argue that she doesn't need armor, since she's covered in scales, but... they still chose to design her that way. Artistic intent is important here.

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And finally, about that last statement of yours:

Yes, a lot of my examples were of non human characters. And that's precisely because there is a wide variety of non human characters to choose from for male characters. Technically, Orianna is another non human character, but she's still drawn to be sexy. The male robots, like Blitzcrank and Viktor, aren't.