As for your point that if sodomy is considered legal the others will be, that is very close to what he is pointing out. He is pointing out that if the legal foundation for sodomy laws is declared unconstitutional, then those other laws can be successfully challenged as well since they rely on the same foundation.
As for incest, many (if not most) states have laws against incest.
Yup, just like they used to have laws against "sodomy".