Najimi is even more ambiguous than say, Hideyoshi who is at least supposed to be a dude who just ended up getting caught up in this whole joke where traps are their own gender. Tadano is legitimately not sure what lurks within that skirt and Najimi isn't giving a straight answer either.
As long as you don't see the schlong it is in a quantum superposition of being there AND not being there which means it's both gay and not gay at the same time which ultimately is not gay. (I'm not too sure of this reasoning but idc anyway)