@neil

re 5:

you're probably actually asking for pronouns or titles so you can address someone correctly in a invoice or email template. Ask for that directly, rather than asking what shape genitals they have, cos that's freaking weird dude.

Outside of medical or sex work contexts I can not think of a reason you really really need to know someone's gender.

"We need it for the advertising profile" isn't as good an excuse as you think it is. Splitting billions of people in to two buckets isn't an effective marketing strategy.