I have never cheated ON a partner, but was involved with a woman who had a fiance, when I was very young.
I was 16, she was about 26. She was my English teacher, supposedly straight, with a long-term fiance and young daughter. We had a 2-year affair, until her family pressured her to marry him. We ended just before I started University.
In terms of feelings... I will be quite honest. I was only interested in her, I was infatuated and swept away by the illicitness of it all. Despite the complete lack of morality, we actually had an incredibly close, deep bond that far surpassed a purely-sexual connection. I felt no guilt. I felt an absolute disconnect to her partner. To me - he didn't exist. I didn't dislike him or dislike hearing about him; though hearing about sexual moments between them made me uncomfortable and hearing about family time together made me envious. Mostly, I just did not consider him as an entity in my world at that time.
She had moments of guilt and we were quite on/off, on/off, each time she battled those feelings. She would feel it, call an end to us, then we would be magnetically drawn back to each other again. I think she also had denial going on and a feeling of disconnect from reality. I am still friends with her today, 12 years since meeting her, and from what she has told me, the seriousness of our affair didn't really sink in until years afterwards, when her daughter started getting close to the age I was when we started our affair.
I think she probably felt the same way that many people seem to feel when they are cheating on a spouse. She seems to have come up with excuses in her head like "it's with a girl; it's not an affair, it's just an experimentation" and "I am not married yet... when I get married, I'll stop", etc. From conversations over the past few years, she does not seem to feel overly guilty (though she privately may). It is clear to me that she was in a very stale relationship when we met, she had missed out on a lot of exploration as she dated him from the age of 16 and got pregnant by accident (condom break) at 18. I made her feel attractive and alive again and the draw was just too much for her.
I haven't been involved with anyone who has a partner since then and I would never become involved with anyone who has a partner nowadays. I am shocked at myself for not feeling any guilt and can only put it down to the age that I was when it happened - where everything in the teenage world is about ourselves and our experience, even if we are the sweetest, kindest and most polite young people; which I was, really. These days, I would feel guilty and this guilt would stop me from getting involved with an unavailable person again.
Me: (30f) open poly
GF: (40f) My long-term, long-distance partner
Hubby (37m): GF's husband
Garcon (26m): GF's submissive/third partner
“Peace comes from within. Do not seek it without." ~ Buddha