It totally depends on the partners that make up the couple. If they can sort themselves properly, things will be fine. There are some cultures however that make it difficult for the man to be comfortable with earning less than the wife. Some women will naturally become arrogant and less cooperative with the husband. Some husbands too might lose some amount of confidence due to this. I personally will not like to earn less than my wife. The key, though is that both partners should be comfortable financially and understand each other too. Financial issues in marriage are really delicate.