Intersex people account for 2% of the population, that’s as common as red hair. Not to mention the variety of other sex related disorders, such as people with extra sex chromosomes or missing hormones etc. There needs to be a word to describe people who fit somewhere in the middle, because using “they” can be confusing.
Only last week a judge in the Netherlands ruled that people are allowed to have their sex registered as “could not be determined.” There’s also new legislation saying that people should not be asked to fill in their sex, unless it’s relevant. E.g. in a hospital it’s useful to know if a patient is a man or a woman, but that is completely irrelevant for a magazine subscription.
I find there are many questions on forms that are irrelevant. Gender is one of them. I find businesses want to know (like auto insurance companies, banks, and employment applications) as a sneaky form of discrimination. Let's say there are 2 people applying for a job. Usually, the perspective employer can tell by the name on the application if they are male or female. But what if the name is unisex like "Taylor, Bailey, or Dakota"? They will look at gender will be used to save money by hiring a female at a lower salary or they hire a male because they don't think a woman could be / should be an auto mechanic. There are other questions on forms such as are you "married, single, divorced, or domestic partnership" and "Do you have children?" that are also irrelevant. My doctor's or dentist's office do not need to know what my relationship status is. It's not relevant to my health care. If they are concerned that lifting more than 5 pounds will be an issue, then either ask me directly if I have small children at home or assume that I am smart enough to know that picking up a child qualifies as "weight".
Not all intersex people are the same. Also most identify with male or female, just with some variation.