Ms. is intended to be equivalent to Mr., not signifying the marital status of the person using the title. This reflects our modern attitude of not absolutely needing to know whether a woman is available for seduction when she, for instance, buys contents insurance.
I have traditionally fluctuated in my use of Ms., because I like equality, but then I also sometimes want to make clear that I am a Miss because otherwise you get called Mrs. anyway, and I really hate that. I tell call centre people that there's no such person when they ring up for 'Mrs Bailey', or sometimes I tell them my grandma lives in Cheltenham. So my bills and things are a mixture of Miss, Ms. and, lately, Dr.
Now and again, drop-down menus are very restrictive and don't allow you to be Dr., Rev., Lady and so on, and you have to pick a gender-specific title. A friend of mine was recently in this position and had to choose either Mrs. or Ms.
At first, this seemed reasonable. You get the choice: do you want to me Mrs. and reveal your marital status, or do you want to be Ms. and keep it undisclosed? But in fact, this completely undermines the whole point of having the title Ms. as an alternative to Mrs./Miss. It only functions if it doesn't state anything about the bearer's marital status, after all, and if it's used in opposition to Mrs., then it implies 'unmarried', becoming synonymous with Miss. For it to retain its purpose, it has to be the only option (with Mrs. and Miss not available) or the Mrs./Miss system must be available: both options must be present. Otherwise, an unmarried woman has to choose Ms., giving it the same function as Miss. While Miss and Mrs. are a pair of contrasting choices, Ms. has to remain non-contrastive (or contrast only with Mr.).