The male is considered the male because once the eggs (from her ovaries) enter his pouch, his sperm enters the egg. So, because she donates the egg and his sperm enters the egg, he is the he and she is the she!
There is some thought that the point of conception (the sperm entering the egg) happens in the water column and his opening his pouch creates a suction effect to pull them into his pouch. But, when you watch it, it appears she extends her ovipositer (seahorse lingo for female
[hr]
!) into his pouch during the mating ritual. It is amazing to witness.