Caught My Partner Cheating Tonight

I was at home tonight, expecting to meet my casual date for drinks and a movie. But when I got there, she wasn't alone. She was with another guy in the bedroom. They were both naked and he had his cock inside her. The wet sounds of her pussy being filled, the slide of skin against skin. He was thrusting hard into her, her back arching off the bed, her moans loud and clear. I couldn't believe what I was seeing. She looked over at me, eyes widening in shock before a wicked smile spread across her face. "Thought you could have me all to yourself?" she purred. I backed away quietly, heart pounding. Should I call the police or just let them be? Is it normal for someone like her to play this kind of game? Am I being too uptight about it? Letting her get away with something so blatant? What should I do now?