Here's my take. I am not Frodo. I read about Frodo. I am not Luke, or Master Chief, or the little Zelda guy.
I am not MC, MC's Wife is not my wife. If I can watch any porn video where two people I don't know bone (and I have watched my share of them), I can play a game where MC's wife screws around. Or where MC screws around. Neither are reflections on me or my actual desires, they're just entertainment.
In the same way, one hopes that most of the folks here don't actually indulge in an unethical, phenomenally traumatic and emotionally damaging relationship with their family members. They just like to play those games.