I will go out on a limb and say that was part of the entirely optional "Diversity monitoring" section or whatever they call it, that goes at the back. It's never seen by anyone who'll have anything to do with interviewing or hiring you, you can write "suck my dick, you bunch of cunts" in every space and it will have no bearing on your chances of employment whatsoever and at least at my work it's tagged to you by a number, no one who sees the info will ever know your name or anything else about you.
Also, every one of those I've come across has "prefer not to say" as an option for everything, ethnicity, marital status, religion, disability, sexuality, you name it, it's all optional, you can fill it in if you want, it'll help with their figures, but you can leave it blank too.