Yes.
Or it should be.
WHY do people think they have the right to dictate behavior to consenting adults?
MYOB.
are you OK with incest relationships or polygamous marriages?
Oh gawd, here we go again.
How old are you?
For the thousandth time, if it does not harm underage children, I don't really care what consenting adults are doing. I don't care if they're male or female or some of both.
That said, incest, even by adults, is based on the parent having power over the child and polygamy means that only one couple's children are legit. Additionally, there has been at least one morm sect that forced underage girls into "marriage" with adult men.
Unlike you Duck Dynasty fans and devout catholics, I believe children should be protected from predators.