The believer is made holy by virtue of being in Christ. The unbelieving spouse is sanctified by virtue of being united to the believing spouse, not in a manner that makes them a member of the Church but in the manner in which when something holy comes into contact with something that is not, the holy thing has a sanctifying effect on that which is not. The "I, not the Lord" part is in reference to there being no requirement for the believer to stay with the unbeliever, but no requirement against it, so Paul instructs Christians to do what works towards the salvation of the unbeliever because they may yet become a believer because of the believing spouse.
The answer is in the next three verses.
But if the unbelieving depart let him depart A brother or a sister is not under bondage in such cases: but God hath called us to peace. For what knowest thou O wife, whether thou shalt save thy husband ? or how knowest thou O man, whether thou shalt save thy wife? But as God hath distributed to every man, as the Lord hath called every one, so let him walk And so ordain I in all churches.