Gender spectrum?

[Relish in Chaos]

So I'd been browsing this Rule 63 blog here (check it out, there's some quality artwork): http://rule63rules.tumblr.com/

And in the "About" section, the blogger aid this:

The term ?genderswap? is also used quite often, but is technically incorrect. As an aside, I?m making an effort to phase out my personal use of terms that conflate gender with sex or imply that there are only two ?opposite? genders, but it?s also my policy not to change the artist?s comments.

I?ve seen people use ?sexswap? as well, which covers more as far as physical traits go, but still uses ?swap? which many people believe implies only two. ?Spectrumslide? is another alternative that offers the least problems, but few people are familiar with it and it excludes those who may not fall on a gender spectrum.

Now, maybe I'm just ignorant or something, but I fail to see how there can be any more genders or sexes than just "male" and "female". People would point to hermaphrodites as evidence, but isn't that much just a combination of pre-existing sexes rather than creating an entirely new one? And then there are people who apparently don't fit into any gender at all, but that wouldn't be a new gender either, since, well, the clue is in the description: genderless. Neutral. The same way that Atheism isn't a new religion, it's the lack thereof, or that asexuality (look, it has more than one meaning now, so there's no need to dismiss it for "nonsexuality") isn't a new sexuality, it's the lack thereof.

I'm not trying to be negatively discriminative or anything here and I'm perfectly willing to learn, as it's a topic that fascinates me, but I don't see how gender can be on a spectrum the same way that, say, sexuality can. Then again, when I said that intersex isn't a new sex because it's a combination of two existing ones, you could say that bisexuality is just a combination of heterosexuality and homosexuality. But some say that it's a very small minority of people that are completely 50/50 bisexual, as most will always have some kind of preference skewed towards male or female.