Actually, he has. It just hasn't been publicized much. Read Minn's wiki:
--Walt Disney said in a 1933 interview, "In private life, Mickey is married to Minnie. A lot of people have written to him asking this question, because sometimes he appears to be married to her in his films and other times still courting her. What it really amounts to is that Minnie is, for screen purposes, his leading lady. If the story calls for a romantic courtship, then Minnie is the girl; but when the story requires a married couple, then they appear as man and wife. In the studio we have decided that they are married, really."--
And since Mick and Minn ARE the anthromorphic equivalent of Walt & Lillian, who better to know the personal lives of the mice than their creator?