@paul-nebeling is like DTTA says, but Sea Serpent are like dragonnewts since they are human like with dragon features. Dragon in the other hand are shapeshifters so then can be in dragon form or human form (well they still have horns). Apparently all dragon are female at is the reason that they make pact with a knight so the knight will fight with his dragon as a dragon knight, while the dragon became his wife and he must treat her like that. In that way the dragon will be able to have children be dragon (female), dragonnewt or human.
I think that Souta child with Naden will be a male dragon so he can became a dragon king or something like that (well that is just a theory).