Yes, the rigid heteronormativity struck me too. I can believe the Onkali don't have anything equivalent to same-sex relationships since they control everyone's relationships and genetics so closely (and definitely have a sex=reproduction attitude), so you have to wonder how they would feel about it amongst humans.

There's also, as I recall, an assumption that what everyone wants is an identical monogamous lifelong heterosexual(+ooloi) relationship. This prevents not only same sex relationships but polyamory and even short term serial monogamy etc. There's people who clearly wouldn't have stuck with their relationship were it not for the Onkali, but no sense that it goes against their nature (apart from the +ooloi bit), and the people outside the Onkali camps don't seem to have a problem with that aspect.

Have you read any of her other books? "Fledgeling" has a non-gender-specific biological-symbiosis thing, and the immortal shapechangers in "Wild Seed" very occasionally switch to the opposite gender sex (though they're never in a same-sex relationship, even with each other)