7 clean days

Question

Hi Rabbi, I am emailing about the source and therefore obligation of 7 clean days. Apparently the discussion of 7 clean days starts in the gemara, where authorities are discussing, the custom of "some" women who took this upon themselves 7 clean days. it also seems this is to apply to a Zavah, which is likely sexual disease of some sort, and not pertaining to a normal period flow. My question is how did this custom, which doesn't even apply to normal niddah, but zavah, make it to Shulkhan Aruch, and codified into obligatory jewish law. I just don't see how I am obligated to stay away from my wife for 12-14 days. It seems to me upon investigation Din Torah is 7 days of period. Please tell me where my logic is misguided.