What do you make of the story of the Nephilim, or sons? of God in Genesis? Sounds like Ancient aliens! Why is that even in the bible? Is there a more practical 'Christian' explanation for it? Thanks!
There is no simple answer to your question. All we have is guesses.
The Nephilim are mentioned only twice in the Bible, Genesis 6:4 and Numbers 13:33. The two seem to be distinct groups, with only the name in common. Beyond this we have very little to go on. Here is what we know.
Nephilim may derive from the verb NPL, "to go down / fall." Nephilim are the ones who descended (from the highlands? from Heaven?), or who fell (fallen angels?). From an alternate root PWL, it could mean, "the wondrous ones," and in Genesis 6:4 they are famous warriors.
The Septuagint (ancient Greek translation) translated the term as "giants", possibly from Numbers 13:33, where the spies described the themselves as grasshoppers in the presence of the Nephilim. Either they were physically large, or so imposing that others feel small in their presence.
In Genesis 6 they play a part in the "sons of God" having children with the "daughters of men." But which part do they play? Some understand the Nephilim as the sons of God, and some understand the Nephilim as the offspring of these unions. But who are the sons of God and daughters of men? Are these supernatural males and human females, or are these the sons of Seth (who worship the one God) and the daughters of Cain (who do not worship the one God)?
In ancient Jewish thought it was common to assume that supernatural beings, "angels", were capable of having sex with human females and producing offspring. There is no record in these writings of female "angels", so, the only available women for them were human. Some said that the angels were forbidden from sexual activity, so,when some of them did have sex with human females, they were cast out from the divine presence, and thus became "fallen angels". This is the tradition which we find today, claiming that supernatural beings had children by human women.
Far more prosaic, and possibly what you want, are the worshippers of God, the Sethites, intermarrying with the non-worshippers, the Cainites. If the Sethites are, like Abel, shepherds, and the Cainites are farmers and city dwellers, the Sethites may have lived in the highlands,and the Cainites in the valleys. When the Sethites intermarried with the Cainites, the worship of the one God was compromised, setting the stage for Noah's family alone being saved from the coming destruction.
I hope this is helpful,