Are titles sexist?

The ma’am post reminded me of a conversation I had with mum a few weeks ago. I like to think that you can choose your title. I noticed that when I was a minor, my title was “Miss”, and as soon as I turned 18 it was changed to “Ms”. Mum kept Dad’s surname post divorce because she wanted the same surname as us kids. Since “Ms” generally refers to divorced/unmarried women, I asked if she was “Mrs” or “Ms”.

She told me that legally, she’s “Ms”, but she thinks it’s incredibly sexist to base a woman’s title off her marital status. She doesn’t like that our titles will change as our relationships do, but my brother will always be “Mr”. (Young boys are “Master”).