"Yes"

I say quickly, thinking that my best option here is to pretend that I am more experienced than I actually am.

Millie gives me a naughty look, then rolls over so she is laying on her side, her ass pointing towards me. She still has her hand around my cock, but let's go of it, putting her fingers in her mouth, getting them covered with saliva which she applies to her asshole to lubricate it. Millie repeats this motion to get another set of saliva onto her asshole, then she reaches behind her and inserts her middle finger into her ass, slowly fingering herself with it.

Millie looks over her shoulder at me and says:

"Get your cock lubed Matt."

I instantly understand what she wants me to do, so I spit on my hand and rub the saliva onto the tip of my cock, getting it lubricated.

In front of me, Millie is still fingering her asshole with her middle finger, but after a few more seconds she seems satisfied that she has opened herself up sufficiently and her hand reaches for my cock again and at the same time she shifts her body a bit so that her ass is at a perfectly inviting angle for me.

With her hand holding onto my cock, Millie adjusts her ass so my cock is between her ass cheeks, then she uses her hand to guide it into a perfect angle to penetrate her ass. I can feel the opening of Millie's well lubricated ass against the tip of my cock, and Millie pauses for a while, breathing heavily in anticipation.

Then, slowly, she pushes her ass against my cock, her sphincter first resisting, then it gives way and I feel the tip of my cock sliding into Millie's ass.