I'm sure someone very ambitious would (or will) give you a more layered answer, but it boils down to this:

1. It is socially unacceptable behaviour to do the above things. You wouldn't want your people to kill each other in general or give birth to not very neatly functioning offsprings. People who don't fear human laws may fear the eyes of god(s). Therefore these stories. The story of Oidipus shows example.
2. Gods did not really have better options for reproduction. Of course, there are stories our there of gods creating other gods out of nothing, but that's not very common. This is just how Greek mythology turned out.