Just because a story is impossible to believe that doesn't mean it is not true, the story of the "gods" coming down from heaven and having offspring with human women is part of many ancient mythologies, Sumerian,Egyptian, Greek,Roman, and so many more.This is a reference to a common experience or history that goes back at the beginning of human history.
Why would angels wanna have sex with human women? Many reasons...
The Bible never says that angels are incapable of sexual relations, it sounds like in their materialised condition they can have sex and procreate.God is not allowing cross breeding between different species.See Jude 7 one of the reasons God punished Sodom and Gomorrah is because they went after flesh of a different kind (=Sarkos eteras).
The mingling of different seeds and the creation of hybrids right before the Flood was Satan's plan to destroy makind and contaminate the human race and in this way void Genesis 3:15.No human race no judgement.
So much more to say on this subject...