I have done so once. And I paid dearly for it years later. It was certainly among the biggest mistakes of my.life, and it ultimately led to the worst emotional pain in my life.
As to why..... I don't have a good answer. It was early in my relationship, and an ex that I knew to be great in bed made it clear to me that she wanted to fuck. So, I did it. I knew it was wrong, but I also didn't feel a particularly strong connection with my new GF at that point. At least, that's how I justified it.
The new GF essentially caught me a couple of days later. But, she didn't have any hard evidence, so I just denied it for long enough that she decided I must be telling the truth.
About 4 years later,that GF and I were days away from becoming engaged. That same ex called.me, as she did from time to time (I hadn't done anything with her since the incident mentioned above). I never answered her calls. On this particular night, I decided that I had to get her out of my life once and for all, as marriage was on the horizon. So, I decided to go talk to her at her place of work (a public place), try to nicely tell her that I'm marrying another woman, she can't keep calling me, encourage her to work things out with her husband, etc. I thought this may help her feel some closure, which would allow both of us to move on with our lives.
We talked for about an hour after she got off work, sitting in the front seats of my car. The talk was productive. We left with a hug, and what seemed to me to be satisfactory closure for her. On the drive home, I felt great. It finally felt to me like that chapter of my life had closed, and I was finally going to marry the woman I love. I started planning the exact proposal the next day. We had already come to an understanding that we were ready to get married to each other. I just needed to go through the motions with a ring, bended knee, etc.
A couple days later, the GF and I go to Target in my car. We pull up in the parking lot, she steps out if the car, looks down, and sees the ex's name tag from work sticking out from under the seat. That drive to Target was the last time we ever spent together
I was completely devastated. Frankly, though I've moved on with my life, married a great woman, and nearly a decade has passed, I still think about this monumental fuck up regularly. It had a truly massive impact on my life.