Yes, I think this is one of the top concerns. I am very forthright with investors about the fact that one of my cofounders is my husband. The fundamental question I've been asked (in various ways) is whether or not I have the balls, and we have the kind of relationship, where I could fire him it I needed to (I'm the CEO).
After 5 years of marriage and much of the drama that comes with it (deaths, lost jobs, new cities, near bankruptcy, several months spent on different continents for business) has helped us figure out how to communicate with each other about the most difficult parts of life. It can be a bit tough to convey this in a 60 minute partner meeting though, so I can understand why investors hesitate.