Is it normal that i have sex with a neighbor i don't really know?

About a year ago my girlfriend and I moved into a building together. There are only 25 apartments in the building and is a much sought after place to live. We lucked out in how we got it and have loved every minute of being there. Something happened a little over two months ago that I'm still trying to get my mind around.

Even though there are only 25 apartments in this building, we really don't socialize with the other residents. They have their lives and we have ours. Everyone is friendly, but we're all a fairly private group of people. Because of my job, I'm only there three to four days a week and I'm at home on those days. I see a few neighbors every now and then, but most of them (including my girlfriend) are gone during the day. The couple that lives next door to us are in their late 40s, or perhaps early 50s. He is some kind of doctor, she's a stay at home mom/wife and they are empty nesters.

On one of the occasions that I was home alone there was a knock at the door, which given that it's a secure building and that I really don't know anyone there, came as quite a surprise. I went to the door and it was my next door neighbor, Pam. I've never been one to chase after older women. The whole "cougar craze" happened after my wilder days (I'm 31). However, Pam is quite attractive. She's a petite brunette with large brown eyes. Her breasts are on the small side but they are proportionate to the rest of her fit body. Her ass is very hot and was the first thing I noticed about her at the mailboxes. As attractive as I found her to be, I was in no way trying to get her attention at any time that I've ever encountered her in the building and until that day she knocked on my door, I would've said the same about her.

Standing at the door she had what appeared to be our carpet steamer in tow. Sure enough, she said that while I was away my girlfriend had offered to let them use it in their place. She asked me to thank my girlfriend, I said that I would, and then an awkward pause fell over the moment. I asked her if she would like to come in and surprisingly, she said that she would like that very much.

Well, a brief conversation ensued and without warning, Pam moved in and kissed me. She pulled away and said, "I'm sorry but I've wanted to do that since the day you moved in. I've even thought about asking Natalie (my girlfriend) if she would mind."

I'd never had anything like that happen before and though I was shocked, I was incredibly aroused. Instead of saying anything, I leaned in and I kissed her. We kissed deeply and passionately for what seemed like a half hour. I then pulled both of my hands up into her tennis skirt and began massaging her ass. Pam pushed into me when I did this. In one swift move I spun her around, pulled her panties down and bent her over the kitchen counter. I dropped my pants and slid my cock into her. We never changed positions or moved anywhere else. I fucked her steadily for about 15 minutes. She came twice during those 15 minutes and I did along with her on the second time.

Then, as quickly as she'd dropped in, she pulled her panties up. Gave me a wink, a smile and a pat on the cheek and was gone. From that day on, once a week, Pam has come by at the same time, has said very few words and has fucked me in much the same way. We don't discuss it. We don't ever say much at all. We just have sex, and every time she leaves she gives me that same wink, smile and pat on the cheek.

I know it's wrong and I know I'm going to have to put a stop to it, but part of me really doesn't want to. I think I'm addicted to it because of how very unusual it is to be fucking a woman like that and under those circumstances.

Has anyone ever experienced anything like this?