My 20-Year-Old Son Is Dating His First Cousin—and I Don't Know How to Be His Parent Anymore

My son is 20 and recently told me he's in a relationship with his 19-year-old first cousin. They didn't grow up together and only met a couple of years ago, and while it's legal where we live, I can't pretend I'm okay with it. I've already talked to him about the risks of having children, and he insists they don't plan to, but that doesn't make this any easier to process. I love my son unconditionally and don't want to push him away, yet every instinct tells me this relationship crosses a line I never imagined facing as a parent. Do I accept that he's an adult and keep supporting him, or do I make it clear that this is something I simply can't agree with? What would you do if you were in my shoes?