Using "it" in reference to a person is a very tricky matter. While I suppose it could be used respectfully in a situation where you don't know the gender of a human (namely infants) or an animal, there are significant harmful connotations that have come from the word's use for a person.

Transgender people and other gender minorities have been the target of ignorance caused by many slurs, including the use of "it," including myself. The word is used to degrade the victim by ignoring their gender identity regardless if the abuser expressly knows that their victim's identity and has thus decided to objectify them as something they're not. "It" objectifies the victim as something not worthy of a human's notion of gender, therefore the abuser regardless them as non-human.