Conflicted Fantasy in Detail

So I've been thinking about this for a while now. It's not like it hasn't come up before but... it’s been on my mind lately, more than usual. You know how you just can’t shake something? Like that itch you have to scratch. It all started last week when we were at the bookstore and I noticed this guy checking her out, just looking down at her as she bent over a bookshelf, really taking in her backside. She’s got a tight little ass, always wore shorts or skirts, never pants even if it was cold outside. And there he was, staring straight at it. That's when I had the thought, the fantasy: him coming up behind her and grabbing her hips roughly. She wouldn’t know who it was but I would watch as his hands gripped her firmly, pulling her back towards him. He’d be hard already from looking, feeling her body so close to his. Then, he pulls down her panties quickly, she’s wearing that lace thong she loves and they slip off easily. My girlfriend's legs are spread now, waiting for what happens next. The guy starts entering her from behind without a word. I can see the way my partner’s eyes narrow in pleasure and maybe surprise. Her breath quickens, small noises escaping as he moves into her. I imagine him gripping her tighter, pressing against her firmly until she’s on her toes with each thrust. His balls slap rhythmically against her clit, driving her wild. She wraps one leg around his waist, pulling him deeper inside of her, closer than ever before. They’re both sweating now, skin sliding together in a heated embrace that only gets more frantic. And then he would pull out just as quick, turn her around to face him with her hands on the bookshelf for support, and push back into her hard and deep. The look on her face, shock but also want, those eyes dilated with desire. He’d fuck her harder now that she was staring straight at him, meeting his gaze. I know it's a bit twisted, thinking about my partner like this with someone else involved. But the idea of seeing him take her like that has been playing over and over in my head ever since that day. It’s just a fantasy I have no plans to act on, but still feels so real sometimes. Guess you could say it's part curiosity, part desire for what she might feel with someone else being dominant, but mostly about how hot it would be to watch him take her like that. And every time I think of those moments at the bookstore leading up to this fantasy, I can’t help but get hard myself thinking about all the details.