That must be what they've mutually agreed to in their relationships. If that's not how you feel, do something different in your own relationship.
Here's why I think it's a bad idea:
Intimacy develops through time spent together and shared experiences. I am not saying that one afternoon together breeds an affair, but when one afternoon becomes months of afternoons, coffees, meals shared, phone calls, etc. intimacy develops.
Spending that much time together, it's natural for people to talk. They share their opinions about the news, start telling their stories from high school, and can often find themselves wading into deeper topics over time. All of the sudden this person knows you really well, and you know them. You start to have in-jokes, that "you had to be there" story about the crazy lady who always comes to the diner, and maybe a private lexicon.
And, obviously, if you've been able to stand each other's company for this long and this much, there's an affinity between you. You enjoy each other and have fun spending time together.
So now you have a man and a woman who enjoy each other's company, have an intimate connection, and spend a lot of time alone together. I am absolutely NOT saying that this means anything untoward will develop, but it sure is fertile soil.
When it's avoidable, why let that happen at all?