My startup cofounder is my wife. Things are going splendidly. Generalizations are pretty useless in making decisions for your own life.
I think that most couples aren't capable of running a business together, but I also think we're particularly pragmatic and don't let emotions get in the way of our goals.
I also have found myself becoming MORE creative now that we've been married. That might just be because I didn't move out from my parents' house until a few months before the wedding, but I can say that the broad generalization of marriage destroying creativity doesn't apply to me.
The critical aspect here is that you do not entirely control the temperament of your partner. That is up to her and your luck. Best to not throw away this advice and make use of this knowledge while you can.
Of course, congratulations on your awesome luck.
For example, wingsuit diving is essentially suicidal. http://youtube.com/watch?v=JHlOvhlKPvs
But if you happen to be an extremely talented wingsuit diver, such broad declarations don't mean as much.
Certainly heed advice if you are in no way exception with respect to the matters that influence the issue.
Also, about temperament... I've probably thought about the long term implications of temperament and interaction about starting the business far more than average. So if you shouldn't have gotten married, you sure as hell shouldn't start a company together. If you made a good decision (which sadly isn't too common), you can probably work together really well. Goes back to the main point: context matters over generalizations.