Commentary for Kiddushin 136:7

There is a debate in the Talmud over whether one searches for the reason behind mitzvoth. According to R. Shimon, one does. The prohibition of intermarriage is lest the non-Jewish partner turn the offspring to idol worship. Thus it applies to any non-Jew, not just the seven nations.
But the other rabbis do not interpret the reasons of Scripture. And therefore the verse “Do not marry them” applies only to the seven nations.
They, instead, find another verse to derive the rule that kiddushin with non-Jews is invalid. According to rabbinic interpretation, the Torah allows one to marry a captive woman only after she has converted. From here they derive that kiddushin with her before she converts is invalid.