Yeah, super complicated. : ) I mean, you'd have to write an AU society where the moiety thing is so ingrained that you'd never even think of having sex with someone of the same moiety. I think le Guin's point with this is that in a predominantly heterosexual society, that's how people are assumed to work with respect to having sex with someone of the same gender. So let's have people be bisexual, but replace the gender division with something that would seem arbitrary to us, but is super important to this society...