How does this lesson define physical adultery?
Physical adultery is defined as bringing together that which has no business being brought together. The lesson contrasts this with spiritual adultery, which joins two religions that have no business being together—in this case, Judaism or the Mosaic system with any kind of pagan system.
Les explains that the Bible uses the word "whoring" quite liberally, and it is not so much the physical as it is the spiritual. He defines physical adultery as bringing together that which has no business being brought together. In the spiritual realm, the same principle applies: you are bringing two religions together that have no business being together. In this case, it is Judaism or the Mosaic system and any kind of pagan system. This is what God foretold when He said the people would rise up and go a-whoring.