Many businesses have workplace dating policies in order to address potential conflicts of interest situations or sexual harassment claims. Another reason to consider having a policy is in the event of a relationship where one or both of the participants is married.
Many employers will run into a situation where this is an office affair. This could be between subordinates or a superior/subordinate relationship. Unfortunately, as long as people are people, these types of situations are going to arise.
These types of relationships are distracting, generate all sorts of rumors within the workplace, and ultimately impact productivity. Employers need to be out in front by having a clear policy that is appropriately tailored for their business and circumstances. Companies generally don't have anti-affair policies so in most circumstances the relationship will be governed by the dating policy.
There are a number of different policies that can be employed that range from outright prohibitions (which can be problematic) to a modified policy that requires a consent/disclosure. There is no one size fits all solution but some type of policy is recommended.