In a rapidly fading religious era, why is incest still frowned on?

Really the taboo was a religious one, so why can’t adults get involved if they want to? Contraception is easy, so that’s not an issue. I was just reading about a couple who were brother and sister but who married. They were arrested years later. They didn’t know when they married that they were related, orphans separated at birth. Not the first time something like this has happened.