I agree that your aunt was rude.
How you you normally refer to your aunts and uncles - if you are speaking to your uncle, do you call him John, or Uncle John.
I think if you normally call him 'Uncle John' then suddenly not using 'uncle' is going to seem slightly odd. And even though your fiance knew in general who he was meeting I think it would still have been fine for you to have gone down the 'Fiance, this is my uncle, John Smith, Or, I'd like you to say Hi to John Smith, my uncle"
I also don;t think our aunt has any right to dictate how you speak to, or refer to, any of you relations other than herself. for herself, if you introduce her to your fiance as Mary Jones, and he greets her as Mary, she is free to say she prefers 'Mrs Jones',she is also free to invite him to call her Auntie Mary, but I don't think he would be in any way rude to decline.
I think the issue of acquireng relations by marriage is a fascinating one - I would not automatically think of my (hypothetical) husband's aunts and uncles becoming mine, and would not call them Aunt or Uncle, but would think of the spouses of my aunts and uncles as being aunts and uncles. (what I would call them is more difficult, since I call my blood relatives by first names not titles, so my aunt Sally's husband would be Tony, not uncle Tony, just as she would be Sally, not Auntie Sally.)
I can't quite articulate why this would be - maybe because one is more likely to acquire aunts and uncles by their marriage when you are a child, and calling most adults by some title as a form of respect / acknowlegment of their seniority, whereas new relatives that you aquire by your own marriage are met as adults on a more equal footing?
(In general, I mean - of course there are exceptions)