She happened to let it slip that your turned on by sharing her while talking to her guy friend who immediately asked if that meant she was available for sex.
She confronts you about it as you see her bite her lip in anticipation that you are going to tell her to let him know that she is available if he wants to hook up with her.
I've told my girlfriend this numerous times. She's fucked quite a few men in our 9 years together. And for some reason, after she fucks them repeatedly throughout a week, she comes back to me asking me to forgive her, saying, "I just needed some dick."
My exact words to her have always been, "I never want you to apologize to me for fucking someone else. Never. You are a sexy ass woman. You should be fucked by whoever you want, whenever you want, as many times as you want. I've told you that countless times. I'm not going anywhere. I love you. And I want you to fuck other men. So don't ever apologize to me for doing that. You don't have to. I love when you do that."
But yet, she still sees a reason to apologize for doing it. Maybe that's just her way of opening the door back into my life after she's fucked other guys. Idk. But, I will say this, no matter who she fucks or how many times she fucks them, she always, always, comes back to me. I think her and I both know, I'm the man she loves, and that other men, are just too fuck.