I think you need more time to get used to this (though whether you should really have to is another debate altogether). I am constantly 'married off' in both post and conversation.
At first it annoyed the a*se off me. (I plan, for personal reasons, to always keep my own name) but now, after 10 years and especially since having children I realise that it's the easiest thing for people to do.
There are a zillion reasons why they do it: wanting to not offend you (I think) is v. close to the top of the list (I know! Bonkers), not knowing, guessing, assuming, for ease, considering themselves polite.
This happens more and more especially once you have chidren.
Like I said, 10 years since it began, it's like water off a ducks back to me. I try to view it like this. For people who I don't know and probably won't ever meet again, -what does it matter, I let it go?
For close friends who know I'll remind them of my name once and then make a joke out of it.
For people I will see again but have assumed we're married I usually point out that my name is different.
Sometimes I even marry myself off. (Just on occasions when it's quicker i.e. when it's something to do with my children (who have DP's name) and I don't have time to get into te (dull, dull, dull) detail.