"É um homem."
Well imagine you live in a shared apartment with two other people and all three of you are girls/women. One night you wake up because there's weird noises coming from the living room and you go to the adjacent bedroom to wake your roommate because you're worried. You sit there together listening intently, wondering if maybe your third roommate came home drunk from a party and is therefore making those strange sounds, like she's trying to be quiet but she can't because she's drunk. But then your roommate actually makes out like a whispering voice and realizes that it's not your other roommate who even told you she was going to stay at another friend's house that night, you just forgot she'd told you. Your roommate turns to you and, alarmed, tells you in a hoarse whisper "It is a man!". Keeping your cool and avoiding panic, you immediately call 911, because you're obviously being robbed.
But still, in portuguese you cannot say that It is a man because there is no "it" properly speaking. every noun is either feminine or masculine. You cannot say it is a man because the word for being has the gender implied. it would alway be "he is a man" or "she is a woman." ... unless you are speaking of transexuals, saying "he is a woman" implies that although he looks like a man, it is actually a woman.
In the context of the sentence above, the most important thing is the idea of being, that is connected to a feminine or masculine and cannot be sayd without implying it.
In English (as an American) you would never use 'it' when the pronoun refers to a person with a name. Always 'he' or 'she'. This is a bit of a problem for somebody when they want to be gender non-specific.
'It' sounds so wrong that people try to use 'they' as a 3rd person singular pronoun, like I just did, instead