Mickey and Minnie have never been married onscreen; Walt Disney said in a 1933 interview in Film Pictorial magazine that "in private life" Mickey is married to Minnie. This is a bit of a misnomer for some of us. They appear to be married in some films and in other films Mickey is still dating (courting) Minnie.

Premiering with Mickey in 'Steamboat Willie' in 1928, the two of them have always been seen together as a couple in a life long relationship. People started to speculate about their relationship after they (Mickey and Minnie) appeared in several movies.

Walt Disney made Minnie Mouse Mickey's leading lady for screen purposes. If a film should call for a romantic courtship, then Minnie was the right person for Mickey. But, should the film require a married couple, they could also then appear as husband and wife, insinuating that they had been previously married. The Disney Studio seemed to have consciously decided, after having made several movies that the couple were a married pair. There was no need for them to have any public marriage ceremony or acknowledgment event.

More Info: en.wikipedia.org