Pretty sure the author of this articles wife wanted to fuck other dudes because she found out that he actually has a vagina.
TBF, this quote pretty much exactly captures why the author associated his open marriage with feminism. The idea that a dude who would "let" his wife fuck other dudes isn't a real man is exactly the type of stereotype that particular brand of feminism is against.
In the author's words:
that I realized how my status as a Man depended on a single fact: that my wife fucked only me.
Now I'm not saying I would make the same choices as the author, nor do I think that his article was an especially good representation of feminist thought, but I think there is more than a little truth to the idea that monogamy in relationships isn't only valued because it represents sharing love, but also because it represents status and possession. And to be honest, a marriage where the couple is open with each other about what's going on seems vastly preferable to me than a marriage in which one or more of the partners is cheating behind the other's back.
eta: also, kind of fun, because the author's getting strange too.