Meh, I'm not a huge fan of marriage in its current form as a legal institute, but I don't see anything wrong with open relationships, which is mostly what this article is talking about anyway.
I get what he's talking about with open marriage being a feminist issue within the context of his experience, but it hasn't had that tilt within my experience.
I am in a wonderful, loving, hot, devoted relationship with another woman, and I sometimes have sex with our best friend, who is a man. I've never felt drawn to monogamy. I feel like it's a constraint, and that by being denied any sexual expression outside a monogamous relationship, my sexuality is being judged and sentenced to house arrest. My sexuality is not the keystone to my identity, but it is a part of myself which I wish to affirm, explore and enjoy.
We have clear boundaries. We understand that we all care deeply about one another, but that my relationship with my girlfriend is primary, and my relationship with him is secondary. If he gets a girlfriend, I expect his relationship with me to stop including sexual acts. This is an arrangement we are happy with, but I don't think most people could make it work.
I do know a triad who are in a more widely open relationship. The three of them are devoted to one another, and they also all three will have sex with other people. They are about a decade older than me, and I think the experience and maturity that has come with their age is crucial in making their arrangement work.
Whatever you feel is not wrong, but merely true; I don't think anyone should be made to think that their feelings are wrong. I know that a lot of people, especially younger people, have issues with jealousy, and they need to express that to their partners in a healthy and respectful manner and talk it out.
Open relationships aren't for everyone.
If you and/or your partner are interested in an open relationship, you need to seriously talk about it in a very honest and loving way. It may turn out that it's not a good idea for a variety of reasons, but you will still need to discuss what desires prompted the interest in opening up the relationship, how those desires can be sated and how both of you feel about the decisions you've come to.
Communication, honesty and respect are important for any healthy relationship, but especially an open one.