Secondly, there are many many many ways in which SEX is not limited to 2. There are, for instance, 1 in 1500 people who are born intersex: born with sexual characteristics of both sexes. There are, although they are rare, other genetic variations on the XX and XY. As said, they may be rare, but they still exist. read more
Your question proceeds from a false assumption. Lots of people, especially people who stopped taking science classes in middle school, do believe there are two genders. read more
Go read Complete List of Tumblr Genders (So far); there are at least 300 genders in there, and new ones get invented every day. read more
Ok, so yes, there are people out there that believe there are two genders. Some say gender is a social construct. These people are confusing gender with gender roles. read more