In the Old Testament, it says that gay sex is wrong, but it also says that eating lobster is wrong, and that touching a woman on her period makes you unclean. Do you strictly follow all the rules in the Old Testament?
In the New Testament, one guy (Paul) who says that he saw Jesus in a vision says that gay sex is wrong. Jesus himself is silent on the matter. Why would you take that seriously? What if I said that I saw Jesus? Would you believe everything I had to say?
As a further point, do you have any internal moral compass at all, or do you just refer to a book when you're unsure?
Incest, no, because there's pretty much always a power imbalance between 2 family members, making the potential for abuse massive.
Polygamy, I don't have an issue with. There are quite a few people who practice polyamory today.