In 1 Cor 7:12-13 Paul addresses couples who are already married where one spouse is not a believer. This is not the same thing as a believer choosing an unbelieving spouse. If you are single you are to marry a believer. In addition, why would Paul even have to tell these couples to stay together? I think it is because the believing spouse knew they no longer had the same goals and values making it difficult to relate to one another.