ACTUALLY, you can go on dates with other guys even after you are married. Not every single festival, but there are some that the guy with the highest affection will be with you, even if you're married. One example is the fireworks. Ray (my husband) wasn't even AT the festival, because even though he was at 10 hearts, so were like 6 other guys that I got there before him. (I got them all up to 10 hearts, and saved a master file so I can go back and marry them all.) So the game was considering Joe my biggest "fan" or whatever. We watched the fireworks together.
But it doesn't have any practical uses. You are still stuck with Dan everyday. But a little bit of time away from him is nice. XD ---