It's not really an agenda to leave the option open - people not identifying with male or female (or fluidly between the two) doesn't harm anybody. Surely for it to be an agenda, it has to be putting somebody else out?
Same with the Last Name field. I happen to know someone who doesn't have one. Why isn't he accommodated?
How about colour blindness? Why isn't there a colour-blind accessibility option in the sign up form? Is making colour-blind people's options the same level in the UI as regularly-sighted people a bad thing?
I think that my point is made.
Forms of address --- Mr., Mrs., Miss., Ms., and often professional titles (Dr., in German Ing. (engineer), esquire (lawyers), Reverand, etc.) is at least fairly common if not entirely standard practice.
I suspect, again, some motivation on the part of a requestor. A magazine's circulation department, for example, might want to know the number of lawyers and doctors among its subscribers as a proxy for advertising value.
Many business information request forms provide detailed rosters of who you are, what you do, and your company title. For similar reasons, I suspect.
(I also feed those bogus information as a matter of course.)
Having a separate first-last name is often brought up as a UX failure for exactly this reason, and UX designers often design websites so that colourblindness doesn't hamper usability (in my experience working with designers at big companies). The Christmas one isn't as much of an issue because AFAIK very few people are meaningfully put out by seeing Christmas decorations on websites from majority-Christian countries. In other countries these things often /are/ turned off depending on cultural sensitivities.
We should be trying to design our processes to fit the world around us, not rejecting the parts of the world we don't like.
The obvious solution is to just not use pronouns - it's a messy part of the language that is currently in flux, so why wade in?
Obviously, some sites are dedicated to these issues, so they can justifiably ask on sign up, but if you don't _need_ to care about people's personal details, better not to ask at all.