I don't see what argument you can possible make for why say transsexuality shouldn't be considered a psychiatric disorder but being an "other kin" should.

How about the fact that everything we know about ontogeny suggests that gender of a child of human parents should be more fluid than its species, since the determination and development of gender-typical physiology in utero is complex and multivocal? There are ontogenetic factors (insufficient uptake of testosterone, for instance) that might lead to a child with male-typical sexual organs but more female-typical neurological features. There aren't any analogously complex species-determining processes involved in the development of a child.