I have laid claim to my H, and we have had sex pretty much every day since we started our HB. It's been about 7 months since I found out about the A. Mind movies are always there, but I just remind myself that he is with ME, while she is sleeping somewhere else rejected, alone and pregnant. I imagine her there, in her house, wondering what he's doing with me. My H is not sharing a bed with her, and neither am I - she is not welcome in my head while I'm having sex with my H.
The movies are in my head, not his. I, too, imagined their sex like some hot porno movie, but he said it definitely wasn't that. He has assured me that I'm 'better' than she was, which probably shouldn't even matter to me but it does. He said she would have had sex every day, so the fact that it was only 5-7 times over two months was indication that it wasn't really about the sex for him. He gave her sex to keep her coming back, because what he wanted was the validation. Understanding that seems to help in the bedroom, although not so much when we're holding hands or on a date. I'll get there, too, I'm sure.
It wasn't romantic, either. He said he didn't use any creativity and she was a lot more into it than he was, which leads me to suspect she was putting a lot of effort into appearing to be having a great time. He said that, honestly, he didn't care if she was satisfied or not - that was what he liked about it. Just for ego, he wanted to believe he was a super fantastic lover able to attract and satisfy a 21 year old, but he didn't actually CARE about pleasing HER. He didn't have to care about her feelings, if he hurt her, if she felt disrespected, whatever. She meant nothing to him, and so he could be completely selfish sexually. If she actually got off on being treated like that, then she has some issues to work out. Whatever he did with her was the equivalent of a pick-up line in a bar. He probably just did what worked on me, and marveled at how she seemed to enjoy his same-old same-old so much more than I did. That sexual energy wasn't for HER, it was for HIMSELF and how it made him feel to be so selfish and powerful enough to have a woman let him treat her like that. So, that's on him. He has to live with himself on that, and he really does feel tremendous guilt and shame for being so reckless with someone else's heart - even though I have my own separate feelings about that.
I just keep telling myself that this was about him, not her - she could have been anybody, with any body type. All she was was different. He is with me now because of who I am to him. She no doubt looks better naked than I do, so I just accept that she's got a better body and I have my husband. I can live with that.
One practical thing we have done is to just go out and have crazy sex other places than our bedroom. I acted like I was the one having the A with him, and I let go a lot of my inhibitions. What is there left to lose, really? Going out with no underwear, getting tipsy and having sex on a picnic table in the rain at midnight... whatever. Now if I have mind movies, it's a LOT easier to replace those with real memories of our own escapades.
The biggest thing that has helped is actually that I also completely stopped worrying about pleasing him. I decided that if I was going to be having sex with him, it was going to be because I wanted to, and I was for sure going to get more out of it than I was putting into it. TMI? 
