The Gods often had their whims of sexual urges for someone other than the mate they had chosen, and normally, but not always, they were the male deities. Sometimes, mortals were the recievers of these pleasures. And, ninty-nine times out of one hundred, they would sire a child.
Depending on the parent, the child would either be normal or gifted. The more powerful the mortal parent in strenght or magic, the more chances that the child will be gifted. Sometimes, though, demi-gods were the earth-bound children of the Gods, such as Kusanagi and the water dwellers. But some didn't even know who they were.
Bahamut has many children, despite his draconic appearance, a special one in fact, but that myth is for another time.