Despite your grotesque example, the girl without a leg would be better off alive and without a leg, than with a leg and dead. And Margaux apparently was better off with Peter in her life than without him, despite the problems that came about.
It's true that the girl without a leg is better off, but the question is how comfortable you feel saying the guy's behavior was morally OK (or not so bad) based on that outcome.
With legality and public acknowledgement, we could improve behavior.
I will certainly agree that with flexibility and compassion and less shame around sex we could improve things, but I will not go along with "legality".