Why is everyone so obsessed with knowing everyone else's gender?

[SpringchickenGoldBrass]

The only circumstances when I think someone else really needs to know your gender is if they want to have sex with you or give you medical treatment. I do think this is a feminist issue because it seems to me that the insistence of people on knowing one's gender when it really isn't relevant is a matter of treating men and women differently when it's not justifiable to do so.
Why is my gender relevant when I want to book a travel ticket/buy a book on Amazon/join a web forum or (in the most recent case that's annoyed me) ask for advice on a friend's MH issue?

oh yes, I know; it's only so they can say that they have interviewed x number of ethnic minorities/women/other classifications. With colleges, you have to demonstrate some level of Equal Ops, especially if you're only an affiliated college.