Actually they are. What precisely counts as incest varies, and there have always been people who violate it (like any taboo), but this is one of those things we can truly say exists in every documented human culture.
The real thing is that gods and supernatural beings were not generally considered moral paragons; they were usually more like powerful, capricious spirits. The shit they did was very often framed as fucked up. Alternatively, it was framed as not applicable to human morality, because they are so fundamentally different.
Actually they are. What precisely counts as incest varies, and there have always been people who violate it (like any taboo), but this is one of those things we can truly say exists in every documented human culture.
The real thing is that gods and supernatural beings were not generally considered moral paragons; they were usually more like powerful, capricious spirits. The shit they did was very often framed as fucked up. Alternatively, it was framed as not applicable to human morality, because they are so fundamentally different.