My son is dating his cousin

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 from a legal standpoint he 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.