According to Disney, Mickey and Minnie Mouse are married in their "private lives."
It sounds weird and a little too specific, but Disney really took the time to develop their characters. Mickey and Minnie Mouse are portrayed on screen as just dating.
Mickey Mouse originally debuted in 1928. In 1930, Disney began a comic strip for Mickey Mouse. The comic strip widened Mickey’s world even more, though he was already well known from his cartoons.
The stories then became collaborative with many writers. Having so many writers allowed for great storylines, but sometimes left discrepancies. To explain the discrepancies, it was suggested that the characters be presented as “real” actors employed by Disney.
Walt Disney thought it was a great idea. Walt was asked if Mickey and Minnie were married and was said to have told the press that they were indeed married in their “private lives,” but for on screen purposes they were portrayed as only dating.
In a propaganda film during World War II, Donald Duck fills out an income tax form and checks actor as his occupation. In another film, the characters are seen reminiscing on their experience filming the movie itself.