Also, even in 'adult' cartoons, I think sexual attraction to both sexes is something that a lot of people genuinely can't wrap their head around. Especially, as you say, for males. Add the usual dose of 'eeew gayness, ick' and it just goes one step too far down the weirdness path for many.
Less so women though, as you imply. Its just the lesbian exception to the 'eeew' above kicking in again.
