Indeed. Also, the Sethite argument doesn’t explain the giants.
It also doesn’t explain Genesis 6:9 (KJV):
“These are the generations of Noah: Noah was a just man and perfect in his generations, and Noah walked with God.”
Noah was perfect in his GENERATIONS.
Correct. His line was genetically perfect. Now, the question is where did the nephilim come from after? It appears a second incursion of some sort or perhaps one of Noah’s son’s wives was not perfect in her generations and the gene spread into (perhaps Ham and Canaan’s line)? The giants afterwards are heavily associated with Canaanites.
It says Noah was perfect. It doesn’t expressly say his wife or sons or their wives were though, as you may be pointing out, it is implied. I’m not hostile to the view that one or two the sons’ wives may have been carriers (though some of the other things some people come up with taking this idea out can get a little out there). Giants pop up in the lineage of Canaan and less so in some of his brothers. A few may have popped up in Japheth’s descendants. None are mention in Shem’s.