"When you wage war against your enemies, God will give you victory over them, so that you will take captives. If you see a beautiful woman among the prisoners and desire her, you may take her as a wife."
According to the view that the first intimacy is allowed right away, the Torah saw that man will not be able to control his Evil Inclination, and allowed this act. If so, then anything prohibited by the Torah, once a person can't control himself, should be allowed!
Additionally, why did the Torah promise a punishment of being taken back to Egypt, where nobody will want to buy you, and did not mentioned the other three exiles?
The four exiles correspond to the four letters in God's name. Egypt corresponds to the point at the end of the letter yud, which is special and most important of all the elements in God's name. In Egypt, the Jews have already selected all sparks of holiness. Although in any exile God's presence is with them, to find out the sparks of holiness in the converts, in Egypt there are none left, and thus “there will be no buyers.”
Back to the beautiful captive, this law applies only to voluntary, not defensive wars. And to such wars only completely righteous people could go. Anybody who had any fear of having transgressed any Torah law, such as talking between two prayer sections, was required to leave the army and go back. If so, the evil inclination overpowering them could only be a sign that this woman had unrectified sparks of holiness in her.
By allowing first intimacy right away, the Torah hoped that these sparks will re-connect to their source, and the woman will join the Jewish people. Still, a wayward and rebellious son was likely to come out of this union, because of the unrectified parts in their souls.
Art: Boreas - John William Waterhouse - Boreas