In most states, an employer cannot prevent employees from dating. (However, most states permit employers from halting a romance between a supervisor and direct report).
And remember – 50% of all sexual harassment cases start when the relationship was consensual.
So how to mitigate the issue?
One way is through a so-called love contract, in which both parties acknowledge their relationship is voluntary and consensual. Although it’s questionable whether these contracts are enforceable, it seems desireable for an employer to get something in writing.