We're not seahorses. We're a completely different species and one of the many differences between us and seahorses is that male humans can't get pregnat. Only female humans can. Why QT keep bringing up different species to "prove" how human sex is really a spectrum. Would you say humans can fly without the help of technology just because birds have wings and can fly?

Certain organs are associated with a given sex because those organs have a specific purpose in sexual reproduction. We're an anisogamic species, which means we reproduce through the production of specialized cells with half the DNA (gametes) of different size. Male individuals are the ones who produce small gametes (spermatozoon) and females are the ones who produce large gametes (ovum). Then, a spermatozoon and a ovum fuse to form the zygoto, restoring this way the normal amount of DNA. There are several genes involved in sex determination in humans, but the golden star is the SRY gen. Basically, if this gen is present the embryo will develope as male, if not it will develop as female. As this gen is located in the Y chromosome, XX individuals are females and XY individuals are males. There are some people who have a disorder of sexual development (DSD), but they are rare and they are still either male or female, i.e. they don't produce a third type of gamete.

Another thing, seahorses don't have a uterus. According to your links, the organ where fertilized eggs are carried in male seahorses is called a brood pouch. And we know they are males because they are the ones who produce sperm. Females seahorses produce eggs and later deposite them in the male's pouch.