What is amazing is that over time we become less obsessed with the differences between genders and more obsessed with talking about how obsessed we are with the differences.

Perhaps the obsession is strongest among sociologists?

At which point the human doesn't know whether to laugh at the alien or sigh in disappointment. Poor alien thinks the ability to bear children is a matter of gender. So much for intelligent life.

The part where the functionality of our genitalia is reduced to mere variation between size and effectiveness makes no sense. It's begging for some obvious jokes, but let's not stoop that low. (The truth is it could tenuously apply to assorted genital formation irregularities, most of which are health problems, as opposed to the rare instances where it's only an aesthetic irregularity.)

Possible response to alien:

"no, we have this amazing ability to socially construct identities based on the general norms that all of that variation falls into, so 97% of the time, this question is easy as pie to answer. I find it interesting you say "scientifically" you'd have thousands of genders: with our race we find it very easy to general categorize the male female binary because "science" is not the only way of knowing.

I also question what you mean by saying that some find it easier to bear children than others. For us, there are a whole category of people who cannot bear children at all. not because of defects in the uterus, but a complete lack of a uterus and the corresponding presence of seed-productive organs instead. We find it easy to distinguish these people from those who have defects of the uterus, which they go through much of early and even later life not even knowing this is the case, but with the social expectation that the issue of childbearing is relevant for them thus defining this as a gender socially. We'd never say scientifically, say that there is one gender of fertile uterus possessors, and another gender of infertile uterus possessors. Why would anyone classify those things as separate genders?"

You can find the full version here: http://www.therobinhead.com/gender-roles/

And the immediate response is apparently to question the alien rather than our preconceptions of gender. That's the point flying over your heads, folks.

A friend of mine recently had a similar experience on okcupid, in which a guy insisted she was a man pretending to be a woman and that it would be impossible for her, the faux woman, to prove otherwise. She wrote a great essay about it: http://amandapropst.com/2014/07/08/mansplaining-my-body-misogyny-and-entitlement-in-semi-anonymous-social-media/