Here she is giving her darkest inner sanctum to you, reluctantly. It shows on her face. This is the face of pain, the face of submission, the face of why am I doing this. This is the face of anal sex. She's getting cock right up her asshole, right now.
One thing all guys wonder is if she'll give up her ass. We also wonder if a girl's pussy lips are nice. We try to figure it out, but really it's all guess work--you just never really know what's going on down there.
When is this girlfriend you've been dating, you've been gassing up with octane, going to give up her tight little ass and let you ream her all night long. The longer you're with her, the more of a prude she's becoming. It's like she used sex to lure you to her. Now that she has you, sex is not useful for her.