The reply from Mumsnet said:
Hi all and thanks for your input. We always aim to take users’ views into account when designing our surveys. Previously, we've made changes to wording based on your feedback, and we carefully consider these issues each time. In this instance, we have chosen to include "intersex" as an option because there is a very small minority of people who are biologically difficult to categorise. While we understand that intersex is not a third sex, we felt it was important not to exclude that category when gathering relevant data. We appreciate that not everyone will agree with this approach of course, but we believe this wording is the best way to ensure all users feel able to respond accurately.
So actually, I lie, like cheap carpet. They didn’t suggest that they were advised, they made this choice all on their own, which is worse, IMHO.
(And yes, the it’s/its thing is deeply ingrained and makes me very itchy if it’s wrong.)