Employers generally cannot prohibit employees from dating one another (although you can prohibit supervisors from dating subordinates) - yet 50% of all sexual harassment cases begin when the relationship was consensual.
A non-fraternization policy in your handbook is a start, but generally isn't good enough. More and more, businesses are starting to rely on love contracts as a method to mitigate the chance of problems when the romance eventually turns sour.
Joseph Gagnon, writing on behalf of Fisher & Phillips LLP, says that
Properly implemented and appropriately drafted, love contracts will reduce the likelihood of litigation arising from workplace relationships.To my knowledge, love contracts haven't been fully tested in the courts yet - but Gagnon's outline of what should be in a 'love contract' and its benefits are useful reading.
Here's the full article.