You must have an anti-harassment policy. Regarding language specifically about romantic relationships at work, I suggest including language that requires employees to report any relationships immediately to Human Resources but I would not specifically have a policy on which types of relationships are permitted and which are discouraged. Romantic relationships at work are distracting and it best that employees avoid them. Realistically however, it may happen so you want to have room to deal with that on a case by case basis.
Anti-harassment policies encompass romantic relationships without necessarily spelling out which types of relationships are permitted and which are not. Anti-harassment policies should be signed off on by all employees and should also be included in your handbook. Check your state regulations which may even have posting requirements and language requirements. Even if your state does not mandate anti-harassment training, conducting anti-harassment training at least every other year for all employees protects your business should harassment or work romances occur.