Gender vs Sex tick boxes

[DamsonGin]

What would be the ideal for a site registration page in terms of tick boxes for female / male / any other info you'd like to know about your demographic?

Just wondering as MN has Gender: F/M on the registration page (just noticed as I was going to name change for something). What would you actually want you know for a site like this? I'd think Sex: F/M but am still mostly lurking and learning. Or maybe Gender: F/M/Neither as its a social construct?

Sex female or male aka reality

Gender Endless Text box.
So special snowflakes can write an essay on how they are to be referred to by a third party out of their earshot. Those special he/her/they/zir people.

Serious answer. Just have a sex field. As not everyone believes in gender or has one. Most women I have spoken to, are female because of their biology.