Because IMHO the whole concept could have worked if written well and where was the editor at the firm that published the book to ensure that it was?
Because all the other dragon books published here are set in fantasy worlds modelled closely on European history and I finally agree with nojojojo, that one is old. To say nothing of culturally confining.
Because while westerners are going 'sex with dragons ewww bestiality!' Saiyuuki fandom writes Hakuryuu x Hakkai.
Because if we judge by the same pictorial and textual evidence that supports Gojou x Hakkai, Hakkai x Sanzou, Sanzou x Gokuu, Gokuu x Sanzou, Sanzou x Hakkai, Gojou x Sanzou et alia ad infinitem, then Hakuryuu x Hakkai is *canon.*
And that, m'lud, is the case for the defence.