He knows firsthand because he's experiencing it in their relationship. Before this relationship, he was probably against adultery, Hozier From Eden Song Lyrics Analysis
Here is an "interesting review" that plays with your specific phrasing—framing it as an experience of finding "home" in a place that shouldn't exist for the narrator. He knows firsthand because he's experiencing it in
: Most love songs focus on the beauty of the partner. This one focuses on the unworthiness of the lover. The narrator is astonished that after "slithering" from Eden, someone would still offer them a home. he was probably against adultery