Why just admit that humans too have varying measures of sex appeal from totally sexy to next to none at all? Just as men rate womens' sex appeal solely by appearances, so too do women. Just as women reject guys with low sex appeal with the "I'm sure you're a nice guy but I'm not going to have sex with you", men too reject women based on her sex appeal and not on her personality. Hence sexy bad guy and bad gals get forgiven for almost anything.

After all, In the animal world males have to do some routine or duel to show who will be siring the next generation. Humans are critters too therefore sex and chances of getting it are mostly innate (Alphas will always get heaps, Betas get some and Omegas get nothing) and thus it's pointles to appeal to the logic of "but I'm a nice person" to make up for your lack of sex appeal.