About the newcomers - they had three sexes. There was a third who got involved in the "mechanics" when a couple wanted to reproduce. Their inclusion change it from a form of recreation to procreation and they were honored.
I adapted this (and borrowed from other scifi) with my own runs at tri-genders by having the third gender be the one who carried the child. Again, their inclusion move the activity from recreation to procreation and so they became part of the family. In my culture, the third gender tended to remain as caregiver until the child hit puberty. Then , sometime, they may move on if they have not spawned other kiddies in that family.
But there are a lot of ways i have seen tri-genders work.
Also, Enemy Mine used a solo gender alien.
So, really, to me, this concept for alien or fantasy species well pre-dates whatever modern cultural slants folks may want to paint it as.
Especially with examples of gender swapping occuring in real world animals.