In medical contexts, sex becomes relevant. Elsewhere, they mostly care about gender even if they spell gender "sex" on the form, and there are some valid reasons for caring about gender. The most obvious valid reason for them to care is to please their advertisers, who believe (correctly or incorrectly) that gender is a useful predictor of who will buy what. That's a valid reason for them to want to know, but many folks would agree that it's not a valid reason to insist on knowing. Other sites may just want to be able to tell whether they're inadvertently driving potential readers away by gender, or need to get a rough idea to demonstrate non-discrimination. There are also, of course, a much greater number of stupid reasons to ask. And even when they want to know for a reasonable reason, a "myob" option would be nice, from the users' point of view.
And when they have a good reason for wanting to know, if they only provide two options (either because they've never thought about it, or "because there aren't enough non-binary-gender folks to worry about"), they've got no way of ever finding out whether their assumption about how many of their users are non-binary is wrong.
(no subject)
And when they have a good reason for wanting to know, if they only provide two options (either because they've never thought about it, or "because there aren't enough non-binary-gender folks to worry about"), they've got no way of ever finding out whether their assumption about how many of their users are non-binary is wrong.