It's arguable. While it's completely unreasonable for a business to overwork its employees or not pay them, it seems reasonable to me to expect them to not bring their personal issues to the workplace. Also, I imagine their coworkers feel the same; if you're trying to get work done, and your coworkers are fighting because of their relationship, wouldn't you want them to stop/go away?
If your employees are fighting at work because of their relationship you have bad employees. Attempting to forbid all office relationships is not an appropriate response.
