Responses
Hope this helps.
1. Because of their origin story with the black wolf and the white doe that fell in love and what the white harts and the black haired "wolf" represents.
2.I'm guessing through the other royals that have hetero relations with a white hart. It has not been tackled yet and not sure if it will ever be so I'm just guessing. I think if they accepted same sex mates as long as its with a white hart then they are not expecting for them to really have progeny.
3.I don't think white harts have any magic. To answer this last question, you can refer back to my answer to your 1st question.
Why do the White hearts have to marry Into the royal Family? how do they get children? can a pair of a males get children? How? do the White hearts have Magic, or is just myths and people big imagination.