1.
Daniel Sheinfil asked:
Bkvod Harav
If we see that a Ben Noach can marry his daughter why then was it a problem that Lot's daughters procreated with him?
Kol Tuv
Besuros Tovos
2.
The Kollel replies:
The Ramban says that the girls were Tzenu'os, implying that they were correct in what they did because their intentions were sincerely l'Shem Shamayim. However, the Ramban continues that incest was considered extremely repugnant in those generations, and Chazal disgrace Lot for his actions having been motivated by unjustified lust. (See Ramban to Bereishis 19:32; Masecehes Nazir 23a, at the end of the Amud.)
Shimon Brodie