I have learned for women most of the time that an affair starts off as emotional.
My question is can you explain the process of your affair going from emotional to physical?
I mean like expound on what were the thoughts that allowed you to cross that final boundary?
Was it something the AP said or did?
Was it something that happened with your BS?
I guess what I am asking is what was the final straw?
I can't answer your exact question because my affair started off as sanctioned casual dating (my BF and I were in a LDR), so I was actually within boundaries to kiss the OM. However, it was pretty clear that I wasn't supposed to sleep with him, so I'll try to answer from that perspective. For me, the driving forces were:
1. Power. The OM and I started off with a clear understanding between us that our relationship was temporary. It was his last semester of college, and we lived on opposite coasts. He also was in love with someone else who had rejected him. This theoretically made things "safe" in my mind: both our hearts belonged to someone else, and we had a defined end date, so that would keep a lid on any temptation to overstep.
But there's a common phenomenon where two people in the early dating stage get flooded with bonding hormones, and that's what happened with me and OM. It was new to him, and suddenly, I was his be-all and end-all. I think he really believed he was deeply in love with me (based on how long he kept pursuing me long distance after the A was over), but even if it was just a convincing act to get in my pants, the outcome was the same. I loved seeing myself through that mirror. The more I did for him physically, the more desperately intoxicated he became, especially since I was holding out on actual PIV sex. This wasn't a purposeful tease; I really thought I'd never do it, because that was a bright line in my head. My BF and I were virgins when we got together, but I had done everything else with other guys before we met, so I told myself that those other sex acts were less bad as long as I didn't cross the final line.
2. Guilt. Once it became evident that OM had really fallen for me, I felt guilty about breaking his heart. Before I met my BF, my history was mostly as the dumpee rather than the dumper. This thing with OM was supposed to be a "nobody gets hurt" scenario, and I gradually convinced myself that sleeping with him would be a sort of consolation prize given that my BF was always going to win the competition. I did not want to hurt OM and thereby become aligned with the people who had hurt me. Sex was meant to soften the blow.
The glaringly obvious question is why I was willing to hurt my BF, who was far more invested in me. Part of the answer was total cognitive dissonance; I knew it didn't make sense, so I didn't think about it. But also, I really didn't believe he was going to be as badly hurt as he was. He had broken up with me once before, and dating other people had been his idea, so I thought he was on the path to ending things between us. I didn't have the courage or integrity to address this head on. Instead, I twisted things in my head to fit what I wanted to do. Which brings me to #3...
3. I like sex. I'm never going to pretend that I had to grit my teeth and get through it. I wouldn't have cheated just for sex (although I believe there are some WS who do), but in my mind, sex was a perk, not a duty. It also wasn't the transformative experience that some BS insist it had to have been. It was physically pleasurable and emotionally connected, which understandably makes it plenty horrifying enough in my BH's mind. But it wasn't earth shattering, nor did I expect it to be.
Anyway, predictably and embarrassingly, the "final straw" was OM declaring he was madly in love with me. I could tell for a while that he wanted to say it (how much of that was shrewd calculation and how much was genuine infatuation, I'll never know). In any case, it tipped me into throwing caution to the wind. It had nothing to do with my BF at all, other than the narrative I had projected on him of abandoning me, about which he knew nothing.