I can tell the gender of the baby just by what they have on. But sometimes it's hard to tell if the outfit is unisex.

Not if they are asking, "What's it's name?" or, "Is it a boy or a girl?"  If they say, "TAKE IT AWAY FROM ME!" that make me mad.

Why?

What if the baby had a unisex name?

And parents get offended if you don't know the gender of their baby and they get all black and white about it because they think of you as an idiot or dumb or moron or stupid. Of course they won't say it to your face but they will think it and might go online and rant about it on the parenting forum or rant about it to someone in real life like their friend or family or something.

But I think their B&W thinking is narrow and closed minded. Maybe I will just continued mistaking their baby's genders or ask them if it's a boy or a girl and not worry about offending them because I will know it will piss them off, even if they won't show it. That's how much I despise those people.

I get irrationally mad about people calling babies or children It. They are either male or female, with an identity. I think it's offensive to remove the identity of the person. I went so far as to correct all my baby shower cards that said "It's a Boy!" to "He's A Boy!" I was labelled as irrational, rude and unable to 'give up' on my point once it was made.

Some languages require "it" for child, e.g. German ("das Kind").

I'm usually fairly good at telling what a baby is by looking at their facial features - girls usually have finer features than boys.

It is simply a genderless pronoun. Unfortunately due to the horrible ambiguity of the english language, there is no commonly accepted genderless pronoun specifically for people rather than objects.

I don't really mind that either.

Yeah.  Big red flag with me.  The "it" thing really bothers me.