פירוש על קידושין 42:19

The Torah allows the soldier to have sex with the woman the first time before she converts because the Torah knows that men (soldiers in particular?) have an evil inclination and when they see this beautiful woman they will either sleep with her in a permitted way or do something far worse. Therefore, since priests also have this inclination, they too are allowed to have relations with the captive woman. But the second time, after she converts, is disputed. Rav says that since the first act was permitted, she stays permitted to him. Shmuel says that she is a convert and since priests cannot marry converts, a priest cannot marry this woman.