Therefore, all those people that research their family tree on Ancestry are looking a complete fiction after several generations.
Using DNA they are finding that grandpa wasn’t really their grandpa.
I don’t think this was as prevalent in the past. When people worked at home on a farm for example or when the wife worked in a home industry (like weaving, spinning, sewing, brewing, etc) there were nowhere near as many opportunities for people to meet and cheat.