I don’t have a “source” per se’, but it is directly pulled from my education in bioinformatics, biology, and genetics, which I’m (still) working towards my graduate degree in.

So it’s not “thoughts” so much as “If the criteria is X, I know of Y things that are unique designations by which X can be broken down.”

This isn’t to say the sexes are purely limited to what I listed, but these are the ones I know if if the criteria is “differences in reproductive capabilities.”

And you’re right, biology isn’t so nice and neat as people want to pretend it is. There is no way I know of, scientifically speaking, to break down humanity into 2 (or even 3) clear-cut distinct sexes.

Well… I say that.. but there is one way I have come across. “Instigator” and “Instigatee”, basically the person who asks for sex and the person who is asked for sex. And if you look at this this way, there’s a lot of interesting psychology and social dynamics that can be involved with sex. However, by this definition, a person’s sex can change a dozen times in a night in a group-sex scenario.