I'm going to respectfully disagree with Stevesn. With all the trauma interwoven in your sex life now, I think promising to be up for sex at a specific time is priming you both for failure. One or both of you is highly likely to be forcing yourselves into it, which just builds more hurt and resentment.
I want to be clear that nothing I'm about to say is an excuse for cheating. Cheating is always a choice. It is often an avoidance technique for facing something else that's going on inside the WS. We can be sympathetic to their trauma and still be furious and devastated at how they chose to cope with it. As has been observed here many times, whatever a WS's trauma, there's always someone else who experienced the same thing and didn't cheat, just as there's always someone who experienced it and didn't cope with it by abusing substances, or inflicting physical/sexual violence, or any of the myriad ways that hurt people hurt their partners.
The problem you have, from my viewpoint, is that the very thing you need from your WW is the same thing that caused her trauma: an expectation that she needs to offer sex to prove something to you. I am not blaming you for feeling this way. It is, as sisoon says, an entirely natural and valid reaction to an affair. That feeling nevertheless does not exist in a vacuum. It interacts with your WW's trauma in a way that could be very damaging for both of you.
Depending on how the sexual interaction with her father occurred, the expectation of proving love by offering sex may be exactly the message your WW got from him, explicitly or implicitly. As a CSA survivor myself, I can tell you that it's not always a black-and-white case of an adult who forces sex on a terrified child. Grooming can create very conflicting feelings. I was young and just waking up to my sexuality. My predator realized that and abused his position of trust to make me feel powerful, like what we were doing was a consensual act between two equals who were important to each other. Only in retrospect did I realize what a massive abuse of trust this was. I felt angry at him, and then I felt disgusted with myself because I hadn't been terrified. I had been flattered. I sought it out. If he was in his basement workshop alone, I went there alone too, on purpose. Surely, I thought as an older teenager when the memories resurfaced, that meant I was just as complicit as him? What kind of victim indulges her abuser? I had so much guilt mixed in with the anger.
My mother was sexually abused by her father. Her situation sounds like it may be even more like your WW's. My grandmother was institutionalized, and at age 10, my mom was basically promoted to housekeeper and nanny. It didn't occur to me until later that she might have been promoted to substitute wife as well. Unlike me, she was powerless to stop it. I believe she worked it around in her head to where it was a terrible act but forgivable because of the stress her father was under. Because she was trained to make excuses for him, she made excuses for my abuser when I finally told her (many years later) what he had done. I never felt such betrayal as that, the knowledge that she wasn't angry on my behalf, but immediately tried to find a way to take the blame off him. It wasn't until I learned what happened to her that I was able to understand and forgive her.
Anyway. My point is that it is not realistic for either of you to approach sex as if it exists in a vacuum independent of your own traumas. Whatever your WW was doing with the AP, it was deeply unhealthy and had nothing to do with him personally. It was about suffering and avoidance and power and possibly grieving, all wrapped up in sexual dysfunction like a ball of barbed wire. You feel understandably betrayed and broken, on a roller coaster of drawing close and pushing away. If you don't want inauthenticity -- and I believe you that you don't -- then you can't expect to set all that aside and promise to have passionate, validating sex at 3:30 tomorrow afternoon. It is going to be a lot of trial and error even if you are both on the same page about wanting it to work. Neither of you have that kind of control over your triggers. I doubt anyone could.
One last piece of advice. My H suffered greatly from mind movies. Sex between us was a crapshoot. Sometimes it would start off hot and passionate and connected, and then a MM would hit. He tried to hide it from me because it was so important to him to make sex work, both for himself and out of fear that I'd stop trying because I was hurt by rejection. Over time, we learned that we both had to accept the potential of sex being bombed by a trigger. It became an opportunity for him to process the trauma and for me to support him, as well as an opportunity for him to reiterate that he still wanted to keep working through it. Sometimes that meant stopping, and sometimes that meant continuing with an acknowledgement that the dynamic had shifted. But until we stopped white knuckling it, it was like being told not to think about pink elephants. The fear of a trigger made us almost doomed to fail.
I'm so sorry this is happening to you. You didn't deserve any of it, and I'm not telling you what I think you "have" to do. I just want to offer my perspective from my own experience. If you can't or don't want to work though this, you are under no obligation to set yourself on fire to keep your WW warm.
[This message edited by BraveSirRobin at 12:15 PM, Friday, February 3rd]