Actually we do not know any of the characters in question's sexualities.
Yes, Magwyna did not want to marry a bunch of ladies for a wild night of orgies, but since she didn't want to do that with men either. All that means is that she's monogamous, not that she's strictly heterosexual. And we do know that, when pressed about it, she chose to bang one of her wives rather than use her Heroic Diplomacy to try and talk her way out of it.
The same can be said for the women sent here. While we can probably assume that they had no choice in the matter, at least some of the marriage requests were aimed at Magwyna in particular. It is, in fact, fully possible for a far-off chief to hear through traders that the Ymaryn have respected posts and places for people who want to lay with their own gender (and that according to myth their women impregnate each other, somehow), and use that to try and get an arrangement for his oversouled daughter that she might appreciate more. If he could obtain a marriage allegiance, fulfill his familial obligations and at the same time hopefully secure his otherwise doomed daughter's happiness in one swoop, he'd jump at that chance.
Sure, we know that the girls who slept with Yehlmin are at least interested in guys (unless he partakes in some particularly unchiefly behaivours) and really complaining about the son cuckolding some mutually loveless marriages is stupid, especially since the 'husband' both knows about it and doesn't complain in the slightest. But that doesn't mean we can just assume all of the ladies are straight, given our rather limited amount of information in the area.