In California, you can't prohibit employees from dating (although you can prevent supervisors from dating subordinates).
The problem is, 50% of all sexual harassment cases come when the relationship was consensual. And, nearly half of all workers have been romantically tied to somebody from work!
Get a policy in your handbook that puts your procedures in writing. Have your attorney review it, and make sure that all parties in a relationship understand business - from your perspective - always comes first.