Does the Bible always use the word adultery to mean physical sexual sin?
No. Scripture does not always use adultery to mean physical sexual sin. The word is many times ascribed to mixing paganism with true biblical truth, which adulterates the Word of God. In 2 Peter, false teachers with eyes full of adultery corrupt doctrine while fellowshipping among church people.
Les teaches from 2 Peter chapter 2 that the chapter, like Jude, deals with false teachers who will come on the scene in the last days. They come from every direction and leave the average believer confused. In verse 13 they are described as spots and blemishes who feast with you, and verse 14 says they have eyes full of adultery. Les explains that when Scripture uses adultery, it does not always mean physical sexual adultery. Many times the word is ascribed to mixing paganism with true biblical truth—that is, adulterating the Word of God. These false teachers, he says, are masters at that.
Having eyes full of adultery, and that cannot cease from sin; beguiling unstable souls: an heart they have exercised with covetous practices; cursed children: