Am in my thirties, gay, single, with two DCs. I've never been married (my children were born thanks to a sperm bank).
I used to be very keen to call myself "Ms" in my twenties, but now I think "Mrs" maybe sounds more respectable/authoritative...in reality, I don't know that many women who go by "Ms" now (and it's tricky enough to pronounce...)
I find that the GP, Health Visitor etc address me as "Mrs" anyway.
My only reservation, besides not being sure if I am legally "allowed" to call myself "Mrs" if unmarried/not in a civil partnership, is whether it might lead even more people to imagine that I must have been deserted by a partner. When I can be bothered, I often find myself having to explain already that I'm a single mum by choice.