Depends on the situation, but someone cheating once can absolutely regret it and never do it again. I cheated on my now-wife back when we first got together. We were together half a year before we began a 1.5 year long distance relationship, but 5 months into it we were in a prolonged rough patch due to extreme stress she had at the time. I was having strong second thoughts because I saw what I thought was an emotionally abusive side to her I didn't know. I had decided I was going to break up with her the next time we saw each other in person, but I didn't want to do it from another country. I began acting single when I went out and eventually hooked up with a long time off-and-on flirtatious acquaintance,. At the time I justified it as a way to concinve myself that I was done with the relationship so I would actually end it rather than carrying on out of convenience and being too afraid to end it. It was a one-time thing.

But then the situation with my so changed, her stress was over and she changed back into the person I had fallen in love with. She visited and I kept quiet about it, but felt ultra-shitty the whole time. My affair ruined our relationship for me for months. I felt like scum and felt like I was living a lie and didn't deserve her. But I decided to keep the affair to myself rather than burdening her with it, but she found some incriminating evidence of it 7 months afterwards and asked me and I came clean about everything I had done and felt. We had a few long night talks about it and how rocky we had been during that phase and how to handle any similar rough patches in the future, and in the end it brought us closer together. We put it behind us and moved on, I've never looked back and have never considered cheating again.

In some ways I'm glad it happened. Before that, deep down part of me was the kind of person person that could consider cheating if there was 0 chance of ever being caught (though it never actually happened aside from that one time). But now I know now how shitty of a thing it is, how hurtful it is to the other person, and that it would ruin our marriage if I ever did it again even if she never found out. The feeling of regret and scumbagness is something I will never experience again.