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.

presence of certain chromosomes

well, the presence of certain hormones that are usually regulated by some genes on certain chromosomes

Thanks for the explanation! That makes more sense.

Only if you're trying to say they didn't have a gender identity yet as fetuses. That's why it's a meme and obviously a joke. Otherwise it makes an interesting point about how silly it is for cis people to base gender on body parts.

correction: 8 days later, I am editing to state I have since learned biological sex is actually undifferentiated in the first weeks of a pregnancy. This is NOT the same as being intersex, but I think an intersex individual would probably have the best perspective to weigh in on whether the joke in the meme still makes sense, due to overlap in the issues it raises. To my knowledge, I am not intersex, so I'll shut up now.

All cis gendered men are trans would be more correct.

Trans is about how your gender identity relates to their assigned gender. If life begins at conception and you assigned gender at that point, everyone should be assigned female. Anyone who identifies as a man would have a gender that doesn't match their assigned gender and therefore would be trans.