I don't see why it would; the incompatibility is likely on a biological or biochemical level. If I were to guess: vildarii can naturally breed with almost any race, explicitly including leithans, due to the ability to shapeshift both their genitals and their gametes. The former isn't the issue, since leithans and most other major species can fuck each other easily, which means the latter must be what blocks humans, ausar, gryvain etc. from naturally breeding with them. As such, I'd say it's due to the physical structure of leithan sperm and ova being physically incompatible with those of most other races. In th absence of any other evidence, I'd assume they're not genetically incompatible, though that's still a possibility.