If I had to take a guess, I think the innocence ruined came from the piece of shit father's response to the aunt being pushed into the water due to how obviously panicked/caring he was for someone who wasn't his wife and doesn't really have any real reason to care so much. His response was even stronger than the mom's for gods sake, and they're (apparently?) blood related siblings. I assume marrying would confirm all the rumors and suspicions, but at least it'd be putting them to rest instead of letting people continue to speculate and whisper about them for forever. Since the aunt is coming in as a concubine and not a main wife, I think there'd be no need for any fanfare or obvious signs that he wants to marry her, so the episode can pass as man cares about his concubine to be and its not as suspicious (?) instead of man cares about his wife's sister way too much for some weird reason [insert gossip about infidelity]