it sounds like they consider a male seahorse a male because he produces sperm rather than eggs.

The female seahorse drops her eggs off into a male brood pouch, a little pocket the male seahorse has on the front of him that has a placenta in it, and then he fertilizes those eggs and carries the fetuses for a few weeks and then little seahorses flutter out when he gives birth.

there's a video. and it's a LOT of baby flutterhorses

https://animals.howstuffworks.com/fish/male-seahorses-give-birth.htm

Male is the sex that produces the smaller gamete, female the sex that produces the larger

Usually animals are categorized as male and female based on what type of gametes their gonads produce. So male sea horses produce sperm.

~~Not sure how to count the "pregnancy" though, as these are fish and because of the following:~~

The male seahorse is equipped with a brood pouch on the ventral, or front-facing, side of the tail. When mating, the female seahorse deposits up to 1,500 eggs in the male's pouch. The male carries the eggs for 9 to 45 days until the seahorses emerge fully developed, but very small. The young are then released into the water, and the male often mates again within hours or days during the breeding season

From Wikipedia

E: the wiki article goes on to talk about pregnant sea horses, so yeah, they are pregnant and they do get impregnated by female sea horses!

The male doesn't get pregnant. It's like a kangaroo with a pouch to carry the babies.