Or how did it feel to come home after the first time they had sex and watch me so obliviously living a normal Friday night and have a small talk and tell her about my work day?
I think the answer is in brain chemistry, people get a "high" out of this thing, just like if they use methamphetamine, just like so many other things. I don't think those of us who don't get a "high" this type of thing can really understand it, any more than somebody who doesn't drink can understand what it's like to be an alcoholic.
It is destructive, both personally as well as socially in the family, but people still do it searching for that high. I suppose my wife was "coming down" from the high of her adventures with her affair partner when she saw me in the evenings. She was also avoiding me, in many cases, but not always.
As far as accepting and forgiving the lies, that comes with time, and only if there's a lot of work put forth, not only by the wayward spouse, but also by the betrayed spouse. In some cases, it is not forgivable.
Each of us have to kind of figure out where that line is for us. My wife and I worked really hard on this for years, we were in counseling for over four years. She was in individual counseling for even more years so I don't remember how many.
However, she started to backslide in her open communications with me. an ex-boyfriend got a hold of her on Facebook. She didn't mention it to me. They did not meet up, there was nothing inappropriate, but she had had an affair, we had spent years in counseling, there had been a tremendous amount of forgiveness. When I found this out in sort of an unusual way, I had a discussion with her. I made very clear to her that her not informing me that an ex-boyfriend had contacted her showed that she had not learned anything in four years of marital counseling.
I told her that I was fine with her being friends with anyone. But I was not fine remaining in a relationship with her if she was going to be friends with ex-boyfriend and not tell me immediately about them. This isn't about forgiveness, or lack thereof. It is about putting up proper boundaries, proper communications, and maintaining those, not just for a few months or years, but for the entirety of the relationship. She acknowledged the error. Apologized, unfriended the ex-boyfriend on Facebook, and also understood that I was ready to walk because I was not ready to go through the emotional stress again of trying to figure out what the hell was going on in our relationship. Clear communication, open communication, all the time, no excuses.