He watches as you confront her. As you and she push and then snarl.
As your fingers lace in her hair, while hers do the same in yours. Not to pull, but to claim. To keep. And to make it clear that she — your rival, is not leaving until it is settled between you.
Until you have fought, fucked, and somewhere in that struggle decided which of you is better, stronger, and more deserving of his desires.
Not because he is worth such a battle, or because you are not strong enough to throw him away. But instead because it excites you. Meeting another woman as an equal, and then in front of your mutual lover, proving that she is anything but.