First of all, trans men are men, this is M/M. Also, you say you would be okay if it was Simone or some of the other guys you are not that interested but Hiro not being born with a penis is the only one you made a fuss over. I said before you don't have to like anyone, even I'm not that interested in most of the characters actually, doesn't mean it's okay to body shame them like you are trying to do with Hiro.
And "(actual) guys" is a blatant transphobe statement. The whole "really unlikely" argument you are going for is clearly just an excuse you are using to justify your bias. The fact that they are animals doesn't seem to bother you in spite of it being much less possible. Gabriel, also doesn't have the standard set of genitalia. Your "disappointment" is full of contradictions.