Despite the growing popularity of so-called 'love contracts', they're a really bad practice.
While any manager would strongly prefer to eliminate workplace romance, the fact is that they've been going on for generations and will continue to do so.
You walk a fine line between imposing your will on an employee's private life and their performance at work.
In California, it's against the law to prohibit employees from dating, although it's acceptable to prevent supervisors from dating subordinates.
A good 'best practice' is to monitor your employees performance. If it drops off (for any reason - including a workplace romance) - then manage the performance, and not their personal life.