פרק כא פסוק יא
וְרָאִיתָ בַּשִּׁבְיָה אֵשֶׁת יְפַת תֹּאַר וְחָשַׁקְתָּ בָהּ וְלָקַחְתָּ לְךָ לְאִשָּׁה

רש"י (ד"ה ולקחת לך לאשה)
לא דברה תורה אלא כנגד יצר הרע (קדושין כא:), שאם אין הקב"ה מתירה ישאנה באיסור, אבל אם נשאה סופו להיות שונאה (ספרי ריד), שנאמר אחריו "כי תהיין לאיש וגו'" (פסוק טו), וסופו להוליד ממנה בן סורר ומורה, לכך נסמכו פרשיות הללו (תנחומא א)

According to the Midrash (which seems to be based on the word "לך", "for yourself"), the Torah only permits Jewish soldiers to take a yefas to'ar since they would violate a prohibition anyway; best for them to take the foreign woman in a permitted fashion, if at all.

Q: Is Hashem suddenly a Reform rabbi? Since when does the Torah make such accommodations for human weakness or unwillingness to act properly?

A: The Torah is giving us a powerful message. Clearly, Hashem does not want Jewish soldiers to take a yefas to'ar. Since the temptation is so great, however, Hashem grants a one-time exemption in this case, and this case only, for them to submit to their desires. But this exemption comes with a caveat to the soldier: the marriage will be a disaster, and any children from this woman will rebel against the father.

In essence, Hashem is saying the following: "I attest that it is terrible for you to get involved with this woman. This alone should be enough to deter you, but I will leave you the choice in this instance of greatest temptation. If you are not up for the struggle, or if you are one to rationalize your behavior, ignore My advice and see what happens."

The message is indeed powerful. If going against Hashem's obvious wishes will prove so ruinous even in absence of an actual prohibition, how much more will those who challenge and defy explicit commandments ultimately regret it.