I never quite know what this means. People say they won't attend/participate in a gay wedding at all or else only to the extent they're required to, and that, in their heart, they "disagree" or "don't support it" or aren't "celebrating." This disapproval would make more sense if they were talking about a political platform they didn't support, but it's a wedding. And while not everyone is thrilled about everyone else's straight marriage either, when someone says about a straight wedding, "Those two really shouldn't be getting married!" it's a personal comment about those two people, not about all straight people. Normally, the default position is just to be happy about the weddings of strangers. I've never understood what it feels like to object to random strangers being gay and happy, or why (as you say) it is a feeling that needs to be shared with the gay couple.