Since you acknowledged in your earlier post -- and it is stated in the article you linked -- that not all same-sex couples intentionally set out to create "open marriages" (just as not all opposite-sex couples do so), it appears that you apply one standard to all same-sex couples and not to all opposite-sex couples.<quoted text>
Straight wedding vows almost always include "forsaking all others," and I believe most straights intend to be monogamous. Failure to do so is just that - failure. It is not an intentional lifestyle plan like it is with gays, To call an intentionally open same-sex couple "married" is a huge departure from the meaning of the word. It makes such a "marriage" a sham, which is why I wouldn't see such a wedding as a really important event. If gays want traditional marriage, then they should be willing to at least try to maintain the tradition. It doesn't seem ti be happening,
I wonder why that is?
You clearly wouldn't deny civil marriage to opposite-sex couples even though clearly some of them must intentionally enter into open marriages. It's not like gay people invented the concept. I doubt, too, that lesbian couples are as open in their marriages as you might have been led to believe. And while perhaps gay men do opt more frequently for openness than other populations, it cannot be stated as a norm -- there are plenty of monogamous gay men. I happen to be one of them.
So you are opposed to open marriage. Fine. Then advocate for that.
But don't pretend that denying same-sex couples the right to obtain a civil marriage will create more monogamous couples.