I’m a little confused by the question. Do you think marriage is the defining sign of monogamy in a relationship? Since most monogamous relationships I know and have been in are monogamous from shortly after people begin dating. Or being open somehow devalues our relationship since we have sex with other people? Or is it that you think that marriage is nothing more than legal benefits?
For my wife and I marriage is an experience, an expression of the intensity of our bond and pride for one another that we wanted to share with each other and our friends. It’s a special acknowledgement for each other, to share our vows for our feelings for one another, and to share that acknowledgement with the world to recognize that we’re bonded on that deep of a level.
A lot of people who don’t understand being open focus what is assumed to be selfish, as though it’s about people just being greedy, having sex with who they want to, and not caring for their partners feelings which couldn’t be further from the truth. Sometimes I’m not even interested in sleeping with other people while my wife can be busy fucking different guys. Consider the selflessness of supporting your partner do to what makes them happy, to explore new experiences, new people, and to be happy for them because seeing them happy makes you happy. Sleeping with other people doesn’t take anything away from our relationship, if my wife is happy with me, happy with our relationship, and is able to have more enjoyable experiences with other people then she’s just enjoying life and I’m lucky enough to get to share my life with her as well. The heart is more complex than 1+1=love or sex=love, you can experience an abundance of love for and from more than one person, which when being monogamous can be confusing and tricky, but if you can accept each other than it can expand that feeling infinitely. And even with multiple partners there’s nothing that can replace the connection that you have with one person, that stays between you two no matter what you experience with other people.