Rashi on Deuteronomy 21:11/רש"י על דברים כ״א:י״א

1אשת [AND THOU SEEST AMONG THE CAPTIVES] A WOMAN — even if she be a married woman (Sifrei Devarim 211:7; Kiddushin 21b).

2ולקחת לך לאשה [AND THOU DELIGHTEST IN HER,] THAT THOU WOULDEST TAKE HER FOR THY WIFE) — Scripture is speaking (makes this concession) only in view of man’s evil inclination (his carnal desires) (Kiddushin 21b). For if the Holy One, blessed be He, would not permit her to him as a wife, he would nevertheless marry her although she would then be forbidden to him. However, if he does marry her, in the end he will hate her, for Scripture writes immediately afterwards, (v. 15) “If a man have two wives, one beloved, and another hated, etc.” and ultimately he will beget a refractory and rebellious son by her (v. 18). It is for this reason that these sections are put in juxtaposition (Midrash Tanchuma, Ki Teitzei 1).