My Finger in My Sister's Friend

I thought I knew a lot about most things, as 18-year-olds will do, but I didn't have a full grasp on sex yet. I met one of my sister's friends, and found her immensely attractive. She was slim and short, like 4'10," with long black hair. Somehow, we struck up a conversation that turned into a mutual liking. Boldly, I suggested we go for a  walk. She asked where. I replied that there was an unmanaged park a couple blocks away. It was a place of cool, dark forested trails. Just the thing for a hot summer day. So, I drove her over there, and we started walking around. There was no one in sight. I tried to steer the conversation to sexual matters, and she more-or-less went along with it. I put my hand on her shoulder, and she didn't object.  Just that little bit of touch excited me. Thinking maybe I could get a little luckier, I moved my hand to her lower back. Very lower back. Still, no objection. Meanwhile, I brought the conversation around to masturbation. She admitted that she did ti, but didn't volunteer any specific information. My fingers slipped under the waistband of her shorts and panties. Fortunatey, they were elastic, so soon, my hand was on the round globe of the right side of her ass. She didn't say whether she liked it or not, but I took it as affirmative, since she didn't ask me to stop. I was super-excited to feel her pretty, small, smooth round ass in my hand. Here's where I got really crazy: I moved my hand over her ass crack, and as we continued walking, and I could feel the muscles of her butt working with each step, I slowly worked my forefinger between her ass cheeks. Still no objection. I pressed my finger directly against her anus. Still no objection. Instead, she uttered, very quiectly, "Hmmm, nice." It took a while because we had no lubrication, but I managed to work my finger gently into her asshole about an inch. Mind you, we were still walking. Weird, eh? All this was because I was such a nerd at that age, that I didn't know a more elegant way to do things. I so wanted to stop, lay her down in the soft late summer leaves, and fuck her big-time, but didn't know how to steer the conversation. If that wasn't going to happen, at least maybe there was a way I could see and feel her vagina. Maybe I could show her my super-stiff penis, still trapped in my shorts, and she'd want to play with it. Some variation of this seemed inevitable. We continued walking and conversing. I kept trying to bring the conversation back to sex, but it kept veering back to other, more general things. The walking and finger-in-the-ass thing was getting awkward, so I removed it, taking a moment surrepticiously to inspect and smell it. The finger seemed perfectly clean, but smelled out-of-this-world beautiful! Our walk came to an end, and nothing more happened. We said our goodbyes. I was convinced this was the first girl I would fuck. I was sure she'd become my girlfriend. It never happened. I saw her a few times, but she was always with my sister. Somehow I was too shy to ask her on a date. Then my sister went back to college, and that was that. I never saw the lovely young girl again.