Legal loopholes and gray areas do not mean acceptance. They mean that something that is considered unacceptable and morally wrong becomes harder to detect, prove, and prosecute. Anti-incest laws exist to protect victims of molestation and rape. You take away those laws, you take away those protections. This is not the same thing as molestation becoming acceptable, but the same thing as it becoming easier. I am not saying that it makes molestation acceptable. I am not saying that it makes molestation acceptable.
This isn't an issue of commonplace or acceptance because molestation will never become acceptable and never did I argue that making incest legal will make it so. But it would create tons of weird legal loopholes and situations where burying legitimate claims of molestation and rape is easier than it already is.