Is it permissible to marry a girl with whom one has had a sexual relationship, given concerns that she might repeat such an act with others after marriage?

All praise is due to Allah that you felt your sin, but that is not enough. Rather, repentance and turning to Allah Almighty are obligatory, as is seeking forgiveness for your sin. Know that anal sex with a woman is forbidden fornication and a major sin. As for your fear that she might commit indecency with someone else if you marry her, this is a possibility but not a certainty, especially if she repents sincerely and truly, for Allah may accept her repentance. Legally, there is no impediment to your marriage to her if she repents sincerely. Our advice to you is to perform Istikhara (prayer for guidance) and inquire about her. If you feel at ease, then rely on Allah. If you feel unease, then leave what makes you doubt. All of this is contingent on you knowing that she has repented.