When looking at a person, their gender and sex blend together. Every culture has ideas as to what what the gender roles of the male and female sex should be. So when we see a person's sex, we make assumptions about their gender and what their behavior ought to be.
Not sure if this makes any sense:
I see a person's 'sex' as as their biological/genetic sex.
I see a person's 'gender' as their 'cultural' sex.
If that is the case, then it is not hard to imagine how there could be many more variants of 'gender' than 'sex', if people are willing to distinguish between the two. However, the number of gender variants is typically two due largely to societies equating sex with gender. Two sexes, therefore two genders.
But why does there need to only have to be two? "Just because thats the way it has always be done!", some will say. "Unnatural otherwise," others will claim. "Its liberals ruining society blah blah blah bullshit if you allow more," rant others.
But really, why only two? Don't give God as an answer. You have to do better than that. Not saying there shouldn't be only two, but have a legitimately good reason for it if you are going to impose it. Pussification is not a good reason, either. At the other extreme, it can quickly get idiotic when parents want to see their child a special and unique and therefor will attempt to create a unique gender just for them. The two genders could quickly mushroom into 100000 genders.
While I get the idea behind the 'purple penguin,' I still think that gender and sex will forever be inextricably linked. Getting rid of gender specific pronouns is a losing proposition. I'm not bothered by 'some' of what people whine about being 'PC' bullshit (I get that a woman may not want to be referred to as 'Mrs.' or referred to by her husband's last name), but some of it is silly. I use the words chick, lady, or other -man words without a care.
That said, trying to be more accepting of gender should not come at the cost of acknowledging that there are biological differences in sexes as
@The Oi noted. Channeling whatever predispositions one might have into something positive should be the goal.