A.T. Robertson Commentary 1 Corinthians 7:13

"And the woman that hath an unbelieving husband, and he is content to dwell with her, let her not leave her husband." — 1 Corinthians 7:13 (ASV)

Which hath an unbelieving husband (ητις εχε ανδρα απιστον). Relative clause here, while a conditional one in verse 12 (ε τις, if any one). Paul is perfectly fair in stating both sides of the problem of mixed marriages.