My great-great-grandfather was a skeevy person.
I came to this conclusion after finding out that following his wife's death in 1882, he went and married her baby sister. She was nearly thirty years younger, and he had probably known her since she was in diapers. (His oldest child was about four years younger than she was -we think she was in her early 30s, not that that makes it much better.)
In England, at the time, this would have been illegal and he could have been prosecuted for it if anyone wanted to bother. But they lived in a small town in Maryland, USA, where there was no law against it and nothing to worry about but raised eyebrows and wagging tongues.
On the other side of the Potomac, at about the same period, one of my cousins married his aunt. Well, he wasn't exactly a cousin and she was only his aunt by marriage. It came about like this: Cousin Carrie married a rich widower with a teenaged son, and brought her younger sister (call her "Annie") with her into the household. A few years later, when they were both of age, Cousin Annie married her sister's stepson. There was, presumably, gossip about it for a while.
Then there are all the first-cousin and double-cousin marriages, and a few cases of step-siblings who married each other after their families were blended by his father marrying her mother (or vice versa). The old family tree starts looking a bit kinky.