you are saying that if only Ms was available I would then get asked if I was married or not anyway?
No, I'm not saying that. I'm saying that because Miss and Mrs are in common use, and because lots of people are a bit dim, and don't know quite what to do when a woman is neither, they then ring up to find out which of the two main ways of defining a woman anyone 'actually' is. If Miss and Mrs weren't in common use, of course no-one would ring up to ask which you were.
I should imagine that the reasoning is to find out whether you've ever had a different surname. Which they think they can ascertain by your title.
Personally, I'd just put a box in to tick if you've ever had a different surname, but I expect that's me being oppressive and removing choice etc etc.