I have also heard the argument that the sons of God were the descendents of Seth and the daughters of men were from the other men.

I have studied this subject a lot. My conclusion is that there were giants (Nephilim). The fallen angels (watchers) came down and took women for themselves. The book of Enoch talks a lot about this subject and the punishment those Angels got for doing it. I highly recommend reading Enoch it is very cool book

Great question have fun studying it out;)