Regarding incest, I think that the following information is given " Incest is permitted, with various restrictions, between siblings, even the full siblings of a vowed-kemmering pair. Siblings are not however allowed to vow kemmering, nor keep kemmering after the birth of a child to one of the pair. Incest between generations is strictly forbidden (in Karhide/Oregoreyn; but is said to be permitted among the tribesmen of Perunter, the Antarctic Continent. This may be slander.)"
Of course, something might be forbidden but still practiced by some.
Anna, I think that your finding some of the sexual practices offensive is similar to how Genly Ai feels. He may not be offended exactly but he seems very uncomfortable with the gender switching of the people on Gethen.
I'm wondering if the child rearing practices with the children being raised in the clan-hearth of the mother might be similar to those of a Jewish kibbutz or the experience might be similar to being raised by a single parent on Earth.
Many children of wealthy families used to be (still are?) sent to boarding schools when fairly young. Some children spend all day in day care. I don't know if the child-rearing environment on Gethen would be less advantageous for children.