I've had some new thoughts on this, as this is actually a topic I've thought about a lot over the years.
I'm at the point now, where I consider a woman in a committed relationship to be cheating if she acts like she's single. In fact, if she acts like she's "single" all the time, even if she doesn't actually sleep with another man, it's still a disgusting behavior that is probably worse than actually hooking up with another man in a vacuum. Almost all women will do this by default. This is where you absolutely need to set boundaries and expectations. If she doesn't respect the boundaries, end it! It's really that simple. It's not about "mate guarding" or "jealousy" or "insecurity" it's about respect, and this is a nuanced take. It's not that you should care about potentially losing her to another man, rather you should care how she acts, because acting single is massively disrespectful in and of itself, regardless of the end result. If I was dating a girl who wanted to go out to clubs with her single hoe friends all the time, I wouldn't actually give two shvts if she fvcked a man, i'd care about the utter lack of respect and empathy and the sheer audacity for her to think that's acceptable behavior in a commited relationship.
Lots of guys still have this passive attitude where they insist on indifference, but this is the same attitude that the reddit simps have, ie NOT setting boundaries, which is actually based on insecurity over fear of losing her. The point is, you shouldn't ever worry or care if she is sleeping with other man since you cannot control that, but you should absolutely care how she acts around men in general, because you can and should control this with boundaries. This is where you must lay the hammer down while simultaneously showing indifference to how she reacts to it - your ability to walk should be based on indifference of the outcome, not the actual behaviors the lead to the outcome. Disliking the idea of your girl putting herself in environments where her inhibition is lowered and she's surrounded by men who wanna smash her isn't a trust issue - anyone in her shoes could slip up even if she had no real intention of cheating (which is usually what they say, oh I was drunk and one thing led to another, zero accountability). The mere fact that she'd even put herself in high risk situations in the first place is a sign you should not take her seriously, plain and simple.