How many past empires and kingdoms were based/founded on exactly that principle?
Doesn't make it right.
Must we now all eradicate them from the face of our cultural history because they were "gross"?
No, but we must acknowledge that it was not right, and yes:
Times change
But when Smith and Wyman dated, the Middle Ages were kinda already over IIRC. And I remember the press coverage at the time which was "foolish old rock'n'roller courts teenage jailbait" and Mick Jagger mocking Wyman, but the social consensus was that we were not talking about child molesting or abuse. There are 13-year-olds who aren't children any longer and there are 18-year-olds who still are. When Mandy was 18 at their marriage she didn't exactly look like she had left the pram only yesterday, but like someone who had been a young woman for a while.
So you admit we all knew it was wrong at the time ("Jailbait"). That term also disputes this supposed consensus you speak of - I disagree that existed in the 70s (except among randy perv men). No one stood up and said anything, but many thought it was wrong (and said so privately) as evidenced by the taunting (the old school way to let yer mates know they done f***ed up). He WAS criticised for it at the time, decided to ignore that (he was a rock star and could get away with it).
Also as a lawyer (granted you're more biz than criminal or family law) I am appalled that you would bring up the 'mature 13 year old ' argument. There may be some variation from person to person here but not that much and a legal line has to be drawn somewhere. I would also argue that a 'mature' 13 year old is only so due to previous exposure to not sexual abuse per se but at least "sexual interferance."
The only thing s that have changed are that people are not tollerating it anymore and rich/famous people are having a harder time getting away with stuff like this (yes there may be some opportunism here of the blackmail/gold digging variety but don't anyone dare try that non sequitor here because we know this isn't one of those cases, and you don't make laws for the outlying cases but for the norm, which is that a grown ass man has both a power and experience/knowledge advantage over a 13-14 year old girl that can be abused to get sex, and that sex is not usually in the best interrest of the girl). It's not as if he's being prosecuted - the people have the right to call him out on it by protesting/boycotting some stupid f***ing movie. In my opinion, the big picture here is that he got off easy.
Like seriously, I remember being young and the boys using the word Jailbait; they wanted to do something wrong ("don't get caught, dude"), because it was cool and easier than getting laid with an of age chick who could legally consent (similar issue with getting chicks drink later, and you'd get a similar reponse from me); classic abusive tactic. Take off your rose coloured glasses next time. Sorry but one's right to get your dick wet does not trump hardly anything, least of all rational consent laws that have not actually changed in most jurisdictions since Wyman's time.