Yeah, which they're not. It's a societal difference, more or less, is all; Thats why they dress that way. Because its how women are dressed and in-universe men don't seem to really have enough presence in society to matter. I think if a guy did try something, they'd break every bone in his body. They start from more revealing outfits than we'd consider normal, but that doesn't say anything about their personalities. They are by no means whores.
They'd fit for a trainer game still. Also, the IP having sexual content has NOTHING to do with it. It has to do about trainable personalities. After all, Fate/Stay Night was an eroge (it's H-scenes were notoriously bad, but still), but it'd be a perfect trainer.
Echidna, Tomoe, Alleyne, Ymir, Claudette, and Risty (though I have too much respect to do that to Risty .-.) all work well.
I think trainers take two main paths:
Innocent or Inexperienced girls (most QB girls seem to be implied to be virgins, except Echidna, I think), being awakened
Ballsy, OP, tough girls (most of QB cast especially those listed above) being taken down a couple notches.
The first one is the appeal of Katara.
The second one is the appeal of Azula and Korra.
Toph can go either way.
Also, the mention of Korra brings to mind a third path: revenge training. Making a slave out of a girl that tends to be irritating. But that can and usually falls into category 2, as with Korra.