When my wife's family gets together to play games like Pictionary her grandfather won't let couples play on the same team because he says they fight too much. I wonder if investors feel the same way :-)
Loads of successful businesses were made by couples. Cisco comes to mind first.
flickr too if i remember right
Six Apart.
Even if your spouse doesn't do your web design, he/she is in business with you. Its critical your spouse supports this lifestyle.