A wise employer draws up a policy for dealing with relationships in the workplace. This can include advising the potential employer on an application form or during a job interview that you have a partner working for that organisation to an obligation to alert an employer when a new relationship starts as it often can given the amount of time employees spend at work. Few employers choose to ban relationships in the workplace all together as this could be bad HR practice in most circumstances. However in some cases there could be a betrayal of professionalism eg tutor/student, doctor/patient so clear guidelines need to be in place.
Working with a partner can potentially cause all sorts of problems from compromising confidentiality to emotional problems spilling into the workplace if things go pear shaped so a well communicated policy needs to be in place. The policy needs to cover inappropriate behaviour, favouritism, abuse of authority, conflict of interest, etc and consideration should be made to including family members working together. Fair and consistent treatment should be the aim.
Employees should read their contract and employee handbook to understand their responsibilities.