Sex in biology has a clear aim: genetic recombination. Everything in function of the recombination of 2 gametes can be considered sex in Biology.

(I think that only plasmid exchange is non sexual exchange of genetic information but eukaryotes don’t do that) A lof of social interactions in zoology however, we consider as sex or gender interactions in culture.

the messiness of sex, of others, other organisms, is just a perspective.