Quit “shoulding” on Michael Phelps, or anyone else for that matter. Self-righteous attitudes such as yours do not draw people to Christ; they turn people away.
I understand why he did not rush to marry her at this time, and wish them every blessing in achieving a solid marriage going forward. A rigid, shame-based worldview dims understanding and the light of Christ.
God commands us to tell the truth. He was living in sin.