I guess this could also fall into the "My Cheating Heart" category as well -
I married my wife Julie 7 years ago. She was a 34 year old widow at the time (her husband died in a boating accident) and she had a 13 year old daughter, Kristin.
I heard how nasty teenage girls can be to their fathers, let alone a step fathers....but Kristin was always so sweet. I never tried to be a 2nd dad to her, but more someone she could enjoy being with and talk to whenever she wanted. When Kristin turned 14 and 15, of course I noticed how her body was developing, but I managed to keep my thoughts decent. By the time she was 16 and 17 - wow - she was SO cute. I tried hard not to undress her in my mind, but I could not stop imagining how her young, pert breasts would look and feel. I actually ended up being jealous when she went on a date, and I was happy anytime she broke up with a guy. I had no idea why I was feeling that way.
Julie got a new job 2 years ago, which requires her to travel 2 or 3 times a month. Kristin was 18 then, and more of a woman than a girl. The 2nd time Juile was out of town, Kristin and I decided to watch a movie together. I dont even remember what movie it was, but I remember us sitting next to each other, then Kristin leaning into me, and then eventually her puttimg my arm around her saying she was cold.
Ok - I had my arm around my step daughter, and we were just watching a movie. I kept telling myself it was all just innocent stuff. There was a point in the movie were some kid was giving another kid a hard time about being a virgin. Kristin just came right out and admitted to me that despite all the dates she has gone on, she had not gone all the way yet. I asked her what she was waiting for - and Kristin said she wanted to be with someone older that knew what he was doing, and someone she could trust. Then she looked at me and said "kind of like you". And the atmosphere in the room totally changed. I decided to laugh it off and I said something like "too bad I am your step father". And Kristin said that nobody would have to know.
I tried to think of something to say, but as much as my mind was saying "no" - I also knew I wanted her. She did not help matters when she moved up and went in to kiss me.
We ended up making out like crazy there on the couch. I had a raging hard-on in no time, which Kristin felt right away. She asked me to tell her that I wanted her, and I gave in and said 'I want you SO bad". So Kristin told me to take her upstairs and make love to her. And thats what we did.
I was carfeul and gentle that first time - but she did great at taking my size and letting me know when it hurt so I could back out a little. Eventually we developed a good rhythm...Kristin had told me before that Julie had started her on the pill months ago, so I was able to go natural which was amazing.
I wish I could say I was a stud that worked her body for hours, but that first time, being inside her, sucking her perfect breasts, kissing her young lips....omg, I only lasted about 10 minutes. But then we spent time kissing and exploring every inch of each other bodies, and I was ready for a second round. My first load acted as lubrication for Kristin, and I slid in much easier and was really able to bang away inside her. And it was not a perverted older guy screwing his innocent, young step daugher. We were making love....and she said it many times, and so did I.
We fell asleep together in each others arms - for that night - SHE was my wife.
We got up early and had sex in the morning again, and then she went to school and I went to work. Both of us were VERY happy.
Juile was due home at around 6pm that evening - so I decided to leave work early and got home around 4pm. Kristin was home from school, and she took my hand and back to the bedroom we went.

From that day on - we have carried on our affair. Anytime Juile is out of town, Kristin takes her place as my wife in our bed. We dont take too many risks....a couple times Kristin has given mr oral sex while Juile is in the shower, and once Juile had too much to drink and since she was passed out in our bedroom, I had sex with Kristin in hers. And yes, I have arranged it where I had sex with both Juile and her daughter on the same day - but that does not happen too often.

One time Juile called from her hotel to say goodnight to me. She had no idea that her daughter was naked between my legs, sucking my cock as I talked to her mom.

Kristin and I will joke sometimes when she has a date. If she ever gets serious and married some guy, he will have no idea that he will be sharing Kristin with me, her step father, since neither of us want our secret affair ever to end. Kristin even talked about having sex with me on her wedding day, then having me walk her down the aisle. That would be very exciting.

I sometimes think about the "what if" she got pregant. Lord knows I have pumped enough sperm into her, but imagine my wifes grandbaby being MY baby as well...lol.

I should feel more ashamed, but Kristin and I are both adults. I love her mother, but I love her, too.