Dude, i honestly don't know if a guy liking an mtf-trans with a dong is considered gay.
I would prefer said mtf-trans-with-dong over a manly-man-with-vayajay. I consider myself straight. If i HAD to choose which of those only 2 alternatives would be more attractive.
So I'd guess it's more the person/gender than the primary genital.
But what do i know :-)