To wit: the design of the mammal is one wherein the individual is encouraged to procreate through a series of hormonal stimulations, which induce the desire for coitus. Coitus encourages conception, which provides for the highest potential that the species will survive, due to the volume of such instances dictated by the design.
Such requires the male to fertilize the eggs born by the female… therein establishing the biological norm.
Perhaps you're conflating the biological imperative with cultural normality… No one that I am aware of is arguing that cultural normality rests in the illusion common to individual perceptions. To the contrary, the issue, is that the cultural normalization of sexual abnormality, sets aside the certainty that such abnormalities are an indicator of a larger problem relevant to the collective and it's environment. Normalizing such can only lead to the rejection of any sense of such, thus promoting the probability that such would be missed or otherwise dismissed, leading to cultural catastrophe.
In short, it's just a very bad idea.