I know everyone is bored but this is such a stupid topic. Blizzard has made every race before Dracthyr have human sexual characteristics so it wouldn't be surprising if the "dragon people" race worked the same, while at the same time the actual dragon forms of characters like Alexstrasza have basically no sexual dimorphism, they chose to go with the latter because, like Teriz explained, Dracthyr are the closest thing to playable dragons that we will get to.