There’s nothing inherently wrong with fans of opposing teams co-mingling. Even sexing each other up, sometimes that’s okay. For example, just this past weekend, I wore this shirt to the Iowa-ISU game, even with an ISU alumna on my arm*. Hell, there are plenty of married couples who, for whatever reason, root for different sides of a rivalry. These things happen; fandom usually isn’t that important.
“Usually,” as you can probably figure out, is the operative word here. After all, if you’re so devoted to a team that you feel the need to wear their jersey to your wedding day, there’s probably some larger issues with your relationship with reality and the outside world at work here. Marrying someone wearing an opposing jersey, then, cannot be a good move.