I asked my trans son and he said the following:
"Male and female categories in themselves are fine, but making a third for transgender people would suggest trans men aren't really men or that trans men aren't really women - that we're just imitating or are an identity instead of the real deal.
A third category for nonbinary artists would be acceptable as Sam Smith and Demi Lovato would both fit into that category. Cavetown as a trans man would be a male artist and Kim Petras as a trans woman would be a female artist.
Just remember trans men are men, trans women are women, and for future reference it's 'transgender' not 'transgendered'."