When God says an unbelieving spouse is sanctified by the faith of another, He doesn't mean that that spouse is saved, as some people argue. The word means to be made holy, bit does it mean that those to whom it pertains---namely unsaved husbands, wives or children have salvation? Not at all, as that goes against the balance of Scripture.
Take the verse into the context of the entire passage, which is dealing with marriage relationship that is unequal in the sight of God, in the occasion that they were already married before one has come to salvation and the other has not.
It is a passage that assures the saved spouse that God considers the marriage valid and the children are not illegitimate in his eyes, and so no separation or divorce is required.
All the favour the saved spouse would receive from God, the entire household will also be a beneficiary. It's a good deal!