I always find it a bit strange that people despise simplistic "Sunday-school" answers to life, but then when they read the Bible and find messy situations like the one described above, they say, "What kind messed up story is this?" Real life is sometimes messed up. A book that skipped over the messed up stuff would be pretty useless.
Secondly, the claim was that Christianity -- that is, the teaching of Jesus -- has affected European ideas about morality. Lot is further away from Jesus than Jesus is from us.
I think the person you responded to has an inaccurate view of the history of slavery in Europe. But only someone who has never experienced a different culture could say that Christianity has had no effect on the moral ideas of the West.
I am very surprised you are claiming the old testament is somehow not part of Christianity or should now be considered- am I misunderstanding you? I am not a religious scholar, but that's not a view my local priest would support.
Also I fail to see how bringing up Stalin is relevant, you can't even find a communist these days that thinks he was morally upright.
Two angels arrive in Sodom, and Lot shows them hospitality. However, the men of the city gathered around Lot's house and demanded that he give them the two guests so that they could rape them. In response to this, Lot offers the mob his two virgin daughters instead.
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Lot's wife turns into a pillar of salt, but Lot and his daughters escape to Zoar, and end up living in a cave in the mountains. In Genesis 19:30–38 Lot's daughters got their father drunk, and over two consecutive nights had sex with him without his knowledge.