Because 99,99% of them have no clue on what it's in the Bible (for good or for bad). Actually God is even friendly to incest, as in Loth's story.
That's Old Testament and other than the Creation and Moses it's not overly studied, read, or preached, at least at the churches I have attended and that's cross faith. The name is Lot and if anything Lot was Date Raped by his daughters. His daughters, thought all men were dead and it was their duty to procreate, their father being the only male they got him drunk and slept with him.
Now if you dig deeper you will find the sons of the daughters founded the tribes of Ammonites and Moabites, both enemies of the Hebrews. Back when I actually took Old Testament Literature it was put forth that the story of Lot or at least the incest was put in the Bible to justify the reason for the exclusion of the Ammonites and Moabites.
Who gives a damn for the usual egocentric bullshit of the Bible?! God was supposed to know the future, so the "only worthy guy"'s action were unknown to God? So long for omniscience? And who wrote the story in the book of the Jews?
And I love the way his daughters get to be blamed, as usual in the machismo present at Middle East, it had to be the women's fault! Not to mention that Lot was up to give his virgin daughters (who magically turn into incest sluts in that other passage) for the people to not rape the angels...
That goofy part of the Bible is, at least, ancient porn.