Their primary proof-text used to justify rape is the following tradition:
Abu Sa’eed Al-Khudri reported: At the battle of Hunain, the Messenger of Allah, peace and blessings be upon him, sent an army to Awtas and they encountered the enemy and fought them. Having overcome them and taken them prisoner, the companions of the Messenger of Allah refrained from having intercourse with the captive women because their husbands were idolaters. Then Allah revealed the verse:
وَالْمُحْصَنَاتُ مِنَ النِّسَاءِ إِلَّا مَا مَلَكَتْ أَيْمَانُكُمْ
(Prohibited for you) are married women except those whom your right hands possesses. (4:24)
Source: Sahih Muslim 1456.
Interpreted on its own, this tradition might appear to sanction the rape of captive women. Yet the scholarly methodology requires the collation of all texts and commentaries on a given topic before coming to a conclusive rule, principle, or practice. We must see how the companions of the Prophet, their students, and the scholars who succeeded them understood and applied the tradition within the entirety of Islamic teachings.
In reality, we will find that this tradition is a proof against their position. Sexual intercourse was only made lawful with these women after they embraced Islam, which requires consent.
Scholar Imam An-Nawawi comments on this tradition, saying:
وَاعْلَمْ أَنَّ مَذْهَبَ الشَّافِعِيِّ وَمَنْ قَالَ بِقَوْلِهِ مِنَ الْعُلَمَاءِ أَنَّ الْمَسْبِيَّةَ مِنْ عَبَدَةِ الْأَوْثَانِ وَغَيْرِهِمْ مِنَ الْكُفَّارِ الَّذِينَ لَا كِتَابَ لَهُمْ لَا يَحِلُّ وَطْؤُهَا بِمِلْكِ الْيَمِينِ حَتَّى تُسْلِمَ فَمَا دَامَتْ عَلَى دِينِهَا فَهِيَ مُحَرَّمَةٌ وهَؤُلَاءِ الْمَسْبِيَّاتُ كُنَّ مِنْ مُشْرِكِي الْعَرَبِ عَبَدَةِ الْأَوْثَانِ فَيُئَوَّلُ هَذَا الْحَدِيثُ وَشِبْهُهُ عَلَى أَنَّهُنَّ أَسْلَمْنَ وَهَذَا التَّأْوِيلُ لَا بُدَّ مِنْهُ وَاللَّهُ أَعْلَمُ
Know that it is the way of Ash-Shafi’ee and the scholars who agreed with him that it is unlawful to have intercourse with the captive women among the idolaters and other unbelievers who are without a divine scripture unless they first embrace Islam. They are forbidden to approach as long as they are following their religion and these captive girls were among the Arab polytheists who worshiped idols. This tradition and others like it imply that the women embraced Islam and this is how they must be interpreted. Allah knows best.
Source: Sharh Sahih Muslim 1456.
('Sharh Sahih Muslim' is commentaries on Hadiths in book Sahih Muslim. Imam Nawawi's Explanation of Sahih Muslim is considered one of the best commentaries on Sahih Muslim).
Hence, contrary to the practice of Muslim terrorists who rape their hostages, the righteous jurists prohibited intercourse with non-Muslim prisoners of war, just as they prohibited compulsion in the propagation of Islam. According to earlier scholars, forget Rape, as stated above you cannot have mutual sex with Non muslim womens prisoners unless they embrace Islam.
Allah knows the best.