It's a choice of name based on whether you're married or not! And unless you're a cook from Victorian days, it's not just a choice of name, it's an honorific which is directly and solely to do with whether you are married and nothing else.
As I say, I don't think grannies should be forced to start calling themselves Ms!
Mrs for adult woman would be logical - it's short for Mistress, as are all three titles. I don't think I'd opt for it in a ballot because of the married-woman thing, but on logical grounds it's fine.
Anyway, I doubt you will be forced to change! And whilst you get to be Mrs, I will continue to be 'forced' to discuss my marital status every time I fill out a flippin' form!
DP won't, obv. Being a bloke, it doesn't matter. Just for the ladies, it's nice to know if they're hitched or not before you send out the Amazon order, eh?