Should incest between consenting adults be prosecuted?

There are several European countries that do not prosecute incest between consenting adults – and another is considering adopting the same concepts.

There was a man who moved to Arizona some years ago from another state.

The man originally married when he was 18 to his High School sweetheart. By the time he was 20, they had a daughter. Around the time he was 40 or so, his wife died of cancer – and he was devastated – as was the daughter.

Anyway, the daughter went off to college. Attended for 2 years. One summer when she was home, something just…. “happened”. She and her father had a…. “thing” (that’s how they describe it). And they have been together ever since.
They live together as husband and wife (she is absolutely drop dead gorgeous!!!!). They are absolutely 1000% committed to one another. They enjoy with a passion the very same things.

He had a vasectomy. She had a tubal ligation. Each on their own – voluntarily. There will be no kids here.

Are they hurting anyone? No. Yet, society will condemn them (if society found out).

Why should that be?

Anyone got some thoughts on this topic?