Honestly how can two cheaters (my own parents) love each other?

It's been several years since I accidentally stumbled upon my father's old journal and read it. Both of my parents cheated long ago and here they are, still married and happy in their own ways I guess. In fact one of my younger twin sisters is 7 months pregnant now (she's just 19) and they're going to be grandparents for the first time.

What I read was gross. My dad (both then 20 year-olds at the time) literally walks in on my mom cheating with his friend in his own house, on the couch. Basically caught in the act. He break ups and fight/punches the friend, she begs, apologizes and promises to never cheat again. Then he takes her back months later, worked it out through couple therapy, they get married years later and I'm born 2 years into their marriage. Dad still had doubts and does the paternity test twice and it was positive twice. He puts his distrust at rest but his ego is hurt. He still revenge cheats; I was only a toddler at the time. Though he confesses weeks later. They worked it out again; this time with a marriage counselor. They promised to each other no more cheating and that's how it's been since then. They had two kids later on; my twin sisters.

If I'd never read his journal, I would've thought I had the nearly perfect family. I've taken them both out of the pedestal since finding that out. My image of them is forever shattered. I never told them I know. I guess it's not my business after all. Supposedly they do love each other. Both cheated. How is that possible?

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Ever since finding that journal, I never felt comfortable seeking relationship advice from them. I can't understand that logic of loving someone you cheated on. I don't get it.
Honestly how can two cheaters (my own parents) love each other?
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