It isn't black and white and we do have informal mechanisms for dealing with such things. If we take the manager and his younger female co-worker, we can presume there's an imbalance at play and so we would expect that if their relationship came to light that HR would take action and fire them both or just fire him. It's not illegal, but it's widely understood to be gross misconduct and sackable. If it's not enforced or it's overlooked, then we have a problem.

We can either make it law or we're happy with the grey. Personally I'm happy with the grey but few like everyone needs to accept personal responsibility for their choices.

When you give everyone the policies and we give everyone all the information available.... we can't then retrospectively engineer the grey area. We have to presume 'adults' (those 18+) are able to make their own choices and their own minds up and support them and help pick up the pieces if their decisions lead to negative consequences.