DP and i are not married but i am Aunty L to 'OUR' niece & nephew and he is Uncle M to 'OUR' niece on my side.
All of the children were born AFTER we got together and our oldest niece is 6 and has called me Aunty L from the minute she could talk and DP and i had only been together less than a year.

I think it is a respect thing, for me my children call the partners aunty or uncle no matter what (well unless of course like my brothers new GF, she is not AUNTY as this stage)