I'm a regular MNer, but am writing under an assumed name (and using more fake names) for what's about to be obvious reasons.
DP is in fact DH. We'd had a non-legal shindig a year after we met, and told everybody that's that. And part of that was because during my previous marriage I was driven nuts by a few assholes misguided friends who'd refused to accept I hadn't changed my name: that it was still "Ms Psmith" not "Mrs Ahmed", so I really wanted to avoid giving anbody a reason to get it wrong again.
DP and I thought a bit later that it might be both pleasing to his mum, and protecting for me if anything should happen to him, to do the legal thing, so we secretly married, inviting only our mutual best friend and DH's mum as witness. We made it clear to both of them that this was to be kept secret, and that my name would remain "Ms Psmith".
Within days, the old bat made me regret inviting her, making jokes about total strangers asking "which Mrs Jones?" (as if they'd know). And within a few months DP/H and I were getting [holiday] cards from her relatives addressed to "Mr and Mrs Jones".
A few years went by. DP/H in the meantime heard from his sister that MIL had blabbed to her as well, but it wasn't officially General Knowledge. DP/H of course explained why this was, and all the stuff you read above, i.e. to respect my name.
We got invited to a wedding in her neck of the family; I sent hand-written acceptances using our two names. So, I was a but surprised to see my dinner placecard written as not "Jane Psmith" but "Jane Jones".
We didn't let it ruin our evening (although the general white-trashiness of the event was worthy of a whole 'nother thread). We stayed through dinner but left before the disco (my being 8 months pregnant proved useful for escape). On our way out, during the goodbye and pleasantries with SIL, I handed her my placecard and said "I am certain I spelled my name correctly on my acceptance - could you please in due course pass on my disappointment to whomever was responsible for this that they chose not to respect my wishes."
DP/H's next phone call with his mother started with her launching right into "If you're ringing to apologise, you're too late and it won't be sufficient" because apparently I "went ballistic" and "ruined the bride's big day" (we'd said nothing about it to her - indeed, I made sure to tell SIL we weren't going to bother B&G with this!).
We have since had no apology either from MIL or SIL. The only thing SIL had to offer - and this is where you MNers come in! - is "well, since I know you're married, I assumed your name was changed".
AFTER THAT ENTIRE CONVERSATION HE'D HAD WITH HER?!? AFTER MY ACCEPTANCE NOTE IN MY NAME?!? AAARRRGGGHHH!
Moreover, I am now bothered by the thought that if what SIL perceives to be my "married name" (I am gagging even typing this) was used, does that mean she spread the secret further? How many people now know?
I actually felt compelled to look up the legality of it, and confirmed what I'd thought I'd known: that name change is voluntary and through usage - though marriage is one of the reasons one might do it, it is not automatic. There is no legal "name change fairy".
So I know leagally where I stand, but what's general consensus? Looking forward to your thoughts.