It's under the circumstance that ;
1) the patriarchs and ancestors of Israel refused to establish inter-marriage with the Canaanites who worshiped idol gods.
That's why Lot's daughters would like to preserve offspring for Lot. It has nothing to do with sexual desire
2) It may be difficult for a Hebrew man or woman to find another Hebrew for marriage, that's why Abraham has to go a long way back to marry his sister. It has nothing to do with sexual desire but the continuity of a family (the Jews) who is faithful to God.
God thus allows these to happen for the continuity of the Jewish families until their population grows and for the Jews to survive history to bring forward God's messages.
However this is no longer the case in Moses time. At that time, some Jews have many wives purely for their sexual desire as population is no longer an issue. The Egyptians actually killed their children to avoid their population growth. Just as Jesus said, Moses allowed them to have many wives because their hearts were hard.