The Crucifixion of the Unbeliever

If somebody has a gunika, an unbelieving wife, it doesn't mean an unfaithful wife. And so presumably using Greco Raman divorce law, he hasn't mind some Gentile, Gentile Christian wife, employing those rules to divorce her husband. Here comes the puzzle in verse which has spawned so much confusing. In systematic theology is it's been abused in treatments of baptism. It's been used as I hate to say it a bad argument for Peter baptism. Even though I'm a Peter Baptist, why should she not divorce her husband for the unbelieving husband is in a state of having been sanctified?