Indeed, many of the prescriptions and proscriptions of the West Church’s MFP were extra-Biblical. The Tanakh, for instance, allows for plural marriage, cousin marriage, divorce, and concubinage. Although these practices were all rendered taboo in the Western Church’s MFP, the Eastern Orthodox Church (and many other interpretations of Christianity) did not have as stringent or unusual rules regarding sex, marriage, and family structure — instead continuing to tolerate many of these practices that were explicitly permitted and regulated in the Tanakh.
I feel that it is important here to recognize that Christianity itself, coming from the teachings of Jesus on matters like divorce, had different perspectives on these topics from the Tanakh. Those values were ingrained in the faith tradition long before there was a concept of a Western Church. They are not extra-Biblical, unless you are only considering the Hebrew Bible.