[Svartalfiar]

Two genders - male or female - because only two exist.

Babies rarely are born both male and female, but even then, only two genders exist.
In those rare cases, check both boxes, but only based on biology.

Actually, that depends on the language - many have neuter as a third gender, and some include many more, such as inanimate/animate, human/other animal, celestial/earthbound, and so on. English doesn't really have gender - nouns are almost always neuter, unless it's an animate object of which the sex is known, in which case the only gender found in the English language is pronouns like he, him, or her.

Babies are not born both male and female - they are still one or the other. Most so-called 'hermaphrodites' may have both sets or indeterminate sexual organs, but that is a physical deformity, as the chromosomal makeup is still almost always either male or female. And, they are usually (if not always) sterile. The only exception is a very rare splitting of the chromosomal makeup into part XX / part XY, or part X / part XY. The huge majority of abnormalities (Turners, Klinefelter's,. Jacob's, etc) are specifically male or female - a male cannot have Turners Syndrome for example. There are very few people with true hermaphroditism, it's hard to find data (most places that collect this lump any kind of ambiguous sex organs with true hermaphrodites, calling all of them 'intersex') on an actual estimate of the population. The more factual articles I've seen (ie not on ask.com or yahoo answers) place the actual, truly hermaphroditic populace, at somewhere between 1/20,000 and 1/100,000.

[Tired of Taxes]

The word "genders" has been used (colloquially) as a synonym for "sexes" for a long time.

But... OK. Point taken. Correction to my post:

Two genders sexes - male or female - because only two exist.

Babies rarely are born both male and female, but even then, only two genders sexes exist.
In those rare cases, check both boxes, but only based on biology.