The only question that 1Cor7:14 can legitimately be called to answer in the p-b debate is a "qualitative" question. That is to say, if only the "holy" may be baptized, then an infant or other minor child belonging to a believer possesses this quality. Or put differently, there certainly is no "unsanctifed" quality of such an infant that would rationally preclude a baptism.
If one asks then about the spouse and baptism, the proper response is: we have to go elsewhere to determine for any and all the proper recipients of baptism. Certainly, the text here doesn't say exactly the same thing about the spouse that it says about the child.
The relationship to the parent is the producer of this quality, however the quality "inheres" in the child--that is to say, it is his federal holiness. If some family-type connection to an unbeliever effected an unclean or unholy state then this could not be the case; but Paul says this IS the case (and implies that this is obvious, that they know this is the case).
The sanctifying strength of the indwelling HS is superior to the profaning strength of unbelief. It is the same power that could touch an heal the "untouchable" leper. For an ordinary man, such a touch would defile him; in Jesus' case his virtue overcomes the defilement and cleanses it.
There is an evident difference in the text between how the the spouse is spoken of, and how the child is so spoken. The unbelieving spouse is "sanctified" in the relation he or she bears to the believer. We should therefore say that the spouse's federal holiness "inheres" in the relationship to the believer--it belongs to the relationship and not to him. Dissolve the relation, and the sanctity will evaporate.
The CHILD, on the other hand, is said to bear this quality not strictly in the parent but for himself. That is, neither unbelieving spouse is said to be "holy," but "is sanctified" (verbal idea); whereas the child is HOLY (substantive adjective). So, we should say that it isn't so easy to remove this federal holiness, since it is a factor of his person, and not an element of the relationship. But not being strictly a spiritual quality, a child of latter years (no longer "belonging" to the parent in exactly the same sense anymore) may well corrupt this holiness.
