Oh yeah, nowhere have I made a contract about monogamy when marrying. The only thing that was stated is that by getting married, we form a family; nothing about who else that family can entail. [Neither have I promised forever, simply said that I want to commit in the present moment.]
It is simply not realistic to enter into a union, and expect there to be no changes, ever. People change, and relationships need to be adjusted or they will break. That's just life.