I would know. I've been with my wife for 30 years, very deeply committed to each other in the heart. Yet we've been in an open relationship since the beginning.
Sex is fun with others outside of our relationship, and occasionally we have threesomes or foursomes (of course we didn't have that kind of fun once the pandemic began). It didn't weaken our love and commitment for each other - in fact, it made our love much stronger over the years due to the trust we have between each other.
We don't run around fucking others without telling each other about it. It's a lot of fun telling each other about our adventures and it makes our sex life much more explosive between us. It's hot as hell.
You're just a sad little prude that judges others just to make yourself feel better about your sad life.