Why Can Someone with a PhD in Theology Still Commit Adultery?

In this short video, John Piper asks the question and answers it from 1 Thessalonians 4:3-5.

1 Thessalonians 4:3-5

This is the will of God, your sanctification: that you abstain from sexual immorality; that each one of you know how to control his own body in holiness and honor, not in the passion of lust like the Gentiles who do not know God.