My r/s was an affair. He was married and I was in a ltr. I'm trying to think how to answer this question, because I think I should be able to.
On one level the answer is simple: I didn't control my impulses and gave in to my attraction to someone else. But I think you want a fuller answer than that.
My bf (M) and I had been together for 5 years when I met my affair partner (D). When I'd originally met M, I was in a pretty weird place in my life--about 2 years out of an extremely exhausting long term relationship with someone who I did a lot of care-taking for. That person (who is still in my life--I'm still kind of care-taking for him) is not diagnosed as anything but depressed, but my sister in law, who is an NP with extensive experience in psychiatric settings and working with hard core homeless/addict populations, told me he probably has schizotypal personality disorder.
The point is that I was exhausted and had just spent 10 years of my life with a guy who was totally crazy and demanding and dependent. Then I met M. He was older than me by 12 years, extremely intelligent, understood my crazy life very well, seemed like a very stable person, and adored me and thought I had all kinds of potential.
Now, the thing to remember HERE is that I had, essentially, no parenting. I didn't understand it at the time, but M was like a good parent when I needed one badly. I had no experience with someone supporting me emotionally and intellectually, being proud of me, all that stuff. So when M pressed for a relationship, even though I was not sure of my attraction to him, I let myself get swept away. He lived in a different town. Within 10 months I'd moved there with him. Really, my whole life changed radically, and for the better in most ways... .but I was now living with someone who I loved dearly but found I didn't have a lot of sexual chemistry with. And it's hard living with someone under any circumstances, and there were a lot of other problems with the relationship. M became excessively emotionally dependent on me. He was an introvert and only needed me to be happy. He relied on me for all his social needs. I wasn't happy, but (though I did not put it this way to myself) I had come to need him for HIS emotional support, for the things I did not get as a child.
I'm sorry this is so in-depth. But I think there's no quick way to answer your question.
It was an unideal relationship (what one isn't?) in which the attraction was pretty one-sided.
I have a horrible time with decisions.
I had a really messed up childhood, and I have abandonment issues. To paraphrase the report I got from psych testing last fall, one way abandonment fears can manifest is an inability to let go of relationships. That was happening, too.
Also, I have a hard time being loved. It is just hard for me. I have problems. Self-esteem, trust. So that, too, was a stumbling block in my r/s with M.
So that is the backdrop. Then I met D. I was attending a grad program with him, in my home town, which is 3 hrs from where I lived with M. I was staying with family, going home to M every weekend.
I just liked D, thought we would be friends. His background, which had a lot of crazy abuse, both touched and interested me. He was very intelligent, very funny. I felt like we had more in common than anyone else we were in school with, since we were both older and had kind of punk rock youths and had knocked around a lot and had to be self-sufficient. He liked me A LOT. He started calling me all the time. I though he was charming, but I wasn't really sexually attracted to him at first--he was not really my "type" and was very straight/corporate-seeming, despite his background. But I liked talking to him. We were in a situation where we were spending 6 hrs a week in class together. He flattered my work to the skies, asked for my help, really fed my ego. Then he started g-chatting me. That would go on for hours a day sometimes. He was supposed to be at work, but it didn't seem to matter.
Then he said he had a crush on me. I am impulsive, I guess, because I confessed (realizing it as I did), that I had a crush on him as well. We decided it was "harmless." We kept going.
So we were in constant contact on the phone and email and chat at this point, talking about EVERYTHING, finding we had SO MUCH to talk about, UNDERSTOOD each other so well. Mainly I understood HIM so well. This was where the love bombing really kicked in. He could not believe he had met me, wished he'd known me when we were younger, didn't know why everyone wasn't in love with me... .you know the script. When we were physically together, the attraction became really evident. Like shaking, like with a terrible fever.
Should I go on?
We talked a lot about our relationships. I told him I did not want to leave M, that there were too many good things about that relationship. He, on the other hand, told me stuff about his wife that made his marriage sound like a mistake and a dead end. See, the corporate and materialistic values that had turned me off in the beginning? They were HER values, not his. He had capitulated to all her plans and dreams, against his own values, because he was so broken by his terrible childhood and needed her in the beginning to escape his addictions. (Heroin, though clean when he met her, but still on pills and alcohol.) So she was part of pulling himself up out of the gutter. But he had given away too much of himself. Really, his values were MY values.
Now, don't laugh: he said she had BPD.
Now, I am not a cynic, and I also believe he has a good heart. I don't know if he has BPD or what, but I do think he believed all of what he said. I think those feelings were real--even the ones that were projections and mirroring and everything else.
We did not want to let our r/s become physical. I didn't know about emotional affairs, but we were at that point deep into one. We decided we would just not think about the possibility of sex... .until the school year was over, and maybe by then we would be clearer on what we wanted--if we were ready to move on our feelings for each other.
Now, this is something else about me that is hard to really get across without sounding like a total a$$h0le, but... .I just am kind of a free thinker. I mean, I don't just take societal values as received wisdom. I tend to question everything. I also have also placed a very high value on experience. That too was something I grew up with. Look, I'm a child of the sixties and seventies and grew up surrounded by the counterculture. My parents were both nonconformists.
We were in so deep by the end of the school year. I wanted to honor what we had been through. I wanted to not turn from experience. So I agreed to go away for the weekend with him. That was when the affair became sexual.
I was by then a hopeless mess of confused feelings and longing and fear. It was the sweetest feeling I'd ever had, being in his arms, being in his gaze. It felt like the reward for a life of suffering. Still, I had already seen enough

to cause me to keep the breaks on about halfway, but at that point it was downhill. I kept seeing D, but I could not bring myself to leave M, even after he left his wife. He spent another year and a half in love with me (so he said, and so I truly think he believed), but I could not go to him completely. I was paralyzed. I hurt him so badly. I deeply regret all of it.
I don't want to keep going and make this longer. I hope this is somehow helpful in answering your question, which I think is a reasonable one. Again, I don't justify it. I would never ever ever ever do it again. I've paid 100x over for it. But there you have it.
If you have more questions, I would be happy to answer them.
I'm sorry for what you and everyone here went through. In the end, I went through it myself, because he replaced me and lied to me about it until his next r/s was well underway. When he told me about it, I became so crazed with grief that did leave M and tried to get D back, but it was too late.
Steelwork