There is something to be said about being monogamous, but not for all celebrities. Most of us enter into relationships and marriage with the knowledge that the person we are with is our forever person; our only person. However, that is not something that works for all people, including celebrities. Some would much rather be in a relationship that is open; and it works for many.
They know they cannot resist temptation, so they allow their spouses to be open and honest about things like this so that they can be married to the person they love while still doing what they want with other people. Whatever works for them, I suppose. However, you might be surprised just how many celebrities do not believe in monogamy.