It's not the fights that should worry married couples; it's what happens when the battles are over.
Almost all husbands and wives experience conflict from time to time, which is not necessarily unhealthy to their relationships. A verbal spat that stays within reasonable limits can open the windows and give the couple a chance to vent frustrations and release some steam.
The important question, however, is what happens after a fight is over? In healthy relationships, a period of confrontation ends in forgiveness, in drawing together, in deeper respect and understanding, and sometimes in sexual satisfaction.
But in unstable marriages, conflict is never entirely resolved. This is a very dangerous situation, where the consequences of one battle begin to overlap with a prelude to the next. It's a good idea for couples to take a close look at what happens in the aftermath of confrontation.
Are there things that you've said or done that have grieved your partner? Do you need to ask forgiveness for attacking the self-worth of your spouse instead of focusing on the issues that divided you? Are there substantive matters that haven't yet been resolved? Deal with them quickly before they can fester and erode the relationship from within.
The apostle Paul understood this principle clearly. He instructed us not to let the sun go down on our wrath (Ephesians 4:26). That's great marital advice.