I love Pervert Eye and would normally champion the analogy, but it really isn't quite the same thing, is it? In Eye's case, all they wanted to do was to out Napier for professional wrongdoing. It would be entirely right and in the public interest that people know that the Law Society's top honcho had abused his position and flouted his own Society's rules.
In Giggs's case (and of all those past, present, and future sportsblokes who can't keep their flies zipped), apart from a prurient interest in the affairs of the rich and famous, the public doesn't have a right to know about such matters. If they're leaked, we know that the Press will gobble them up and spit 'em out for all to relish, but they are not in the public interest in the same way as knowing which MP took a backhander to run a road through an SSSI, or which heads of sporting bodies promised girls, hotels, and Caribe cruises to other countries' Mps in order to secure certain events.
I think that, broadly, it's got to be that if the issue could affect the rights, safety, security, integrity, or handling of the public, it's in their interest to know - to know how their taxes are spent, by whom, where and why; what's in their food and drink; how safe are their homes, cars, etc. Any persons who might compromise the welfare of the public in any way or who have used public office or their professional standing to gain illegally from those positions - yes, they should all be outed. It is in the public interest in those sort of cases. Fiddling about under the sheets with some bimbo? No - the public isn't affected by such actions.