” If the meaning of marriage is what the popular culture of the moment says it is, then it means nothing. Once redefined, it will be redefined again and again, removing all meaning from the word marriage. “
There is already a 44 year old man who wants to marry his 18 year old daughter (not his by blood, but by marriage)
Then she's his step-daughter. If she's his adopted daughter then they might run into legal issues depending on where they live.
An unadopted step-daughter is not related in any way and would just be a young bride for an older guy and I don't see anything wrong with that at all so long as she freely consents.