Please note that this work is subject to updates and that the most recently updated version will always be the document in my google drive linked to on my Resources page. Please also note that you should not accept any of this at face value and always research any of the information I make available yourself. This is intended to be a simple reference and jumping-off point.
(At the bottom of this post, you can find a list of definitions of the terms used in this post.)
Before anything else, I want to make it clear that I am not going to even attempt to take any moral stance on what is written here, as that is quite beyond the scope of this blog post, and furthermore it would ultimately be pointless and do nothing but hinder my and your arrival at the best possible understanding of the actual content. My goal in posting this is simply the provision of knowledge and the facilitation of discussion in the name of common understanding.
Most people who are aware that Freyr and Freyja were at one point worshiped as gods are also aware that they are siblings. They probably also know that the pair were primarily fertility deities, and then following that, a person might know that this is because they are Vanir gods, and the Vanir are chiefly concerned with fertility.
Less commonly known, however, is that the Vanir were at one point a separate pantheon from the Æsir and other gods that now collectively make up the “Norse pantheon.” If you could travel back far enough in time, you could eventually arrive at a time and place where the Vanir had nothing at all to do with any other Norse deities. This time and place would come across as being strange to the average person–not just because of the difference in time period or location, but also because during this time, the people who worshiped the Vanir were part of a matricentric society that existed in Neolithic Old Europe (approximately 4500-1700 BCE in North-West Europe; the general time period is pushed back by a few millennia as one ventures further south).