About assuming someones sexuality/gender again...

So it is about the question I ask before because I didn't explained it clearly. Okay so it's because someone who was a guy was being talked about liking another guy and I tell those people that we shouldn't assume that he's gay unless he says he is. And then this person told me sounding all annoyed, that why would I assume that he is a straight guy either? That's what makes me confused. Why are they getting mad at me for calling a guy "a guy"? When he is originally a guy in the first place?