And a scrotum seam

You pretty much nailed it.

Also, it gets much more complicated than that; at least 2% of humans are intersex, meaning they’re a mix of male and female, everywhere from outward presentation (rare) to chromosomal (more common).

It’s not just gender that’s a spectrum: sex is, too. I said ‘at least’ because it’s possible for people to go their whole lives without realising they’re intersex. 2% is huge: if you know 100 people, statistically 2 of them are intersex. Most of us know more than that.

The ‘only two genders’ thing is biologically bullshit. Biology is never cut and dried.

eta: for reference, I have a disorder that’s been in the spotlight lately – Ehlers-Danlos – and you’ve probably at least heard of it. You’ve likely talked to someone who has it recently. It occurs in 1 in 20,000 people. Again, roughly 1 in 50 have intersex traits. I wish this had the visibility it deserves. It’s exceedingly normal.

Yeah the Y chromosome doesn't kick in until later in fetal development.

There's actually a surprisingly small amount of biological information stored on the Y chromosome, IIRC the vast majority lies on the X.

About the nipples. You’re right in that we all get them by default. As I understand it the only reason males keep them is because evolutionarily it would take more energy to remove them than keep them. Basically we’re all slackers at the genetic level.

Awesome! Thanks for explaining.