My son (20) is in a relationship with his 1st cousin (19 F). He’s an adult and lives on his own. I obviously don’t like this but still want to support him and he’s technically not doing anything wrong, as it is legal in the state we are in. What would you do in this situation?

Edit: For context, he didn’t grow up with her and didn’t meet her until a couple years ago.

2nd Edit: I’m aware of the genetic risks with children and have cautioned him about that. He says they would not have children together. No need to keep commenting about that.