Your girlfriend is sick of being the composed, controlled High Scholar everyone expects her to be, and decided to to trade her dignity as a scholar for the feeling of a good stretching her apart, and she wants you to see exactly how she's being replaced. By letting Rorothy ruin her right in front of you, she's deliberately destroying the person she spent years building. She's choosing to trade her status and her loyalty to you for the raw, degrading sensation of being filled up and stretched out by a man who represents everything her academic life is not: brute force, physical dominance, and zero restraint.