Again about Reach him or Reach it

by Nestor
'It' can only stand in for an object or an animal of unknown (or unknowable) sex. "You can still reach it' means 'you can still get to the theater'. "You can still reach him' means 'you can still reach John'.

Dear Nestor, you said: “of unknown sex”. Does it means I can say?: "He (I know my cat is a male animal) is good cat…?"
On the other hand, if “it” can only stand in for an object or an animal, why we say (scream) when somebody knocks the door: "Who is it?"
Eladio