What most fundamentalists stress in debate about this is that they feel their definition of marriage is god's definition (one man and one woman) and it's blasphemous to change that. When they consider something sacreligious or blasphemous it's pretty much impossible to get them to discuss actual harms or benefits because god's law trumps all.

For example I have literally begged them to explain in what way they can no longer marry someone of the opposite gender anymore, or how they are obligated to officiate same-sex weddings or accept same-sex members. Or how anyone comes into their existing relationships and tries to change them. Either they tap dance around this and keep returning to "it's MY definition and you can't change it" (or the proxy, "it's god's definition and you can't change it" ) -- or, they express slippery slope concerns, that say the Next Step is to force them to officiate gay weddings and such -- despite that this is in no way what anyone has ever proposed.

Or they conflate things, like, you have the example of a church that decides to rent its meeting hall TO THE PUBLIC -- not just to members -- and is then sanctioned for refusing to rent it for a same-sex wedding. Which is easily solved -- just don't rent it to the public; simples.

So what it comes down to is the usual mis-identification of self with ideas, such that disagreeing with or not supporting your ideas is a direct personal threat -- and/or, they don't understand that they can't sneak their ideology into the public sphere and are surprised when they aren't allowed to.

Put another way, fundamentalists in America have so long had de facto control of society at least at the local and regional level, and have gotten away with running the show as they please for so long, that shifting public sentiment that they can't control is a direct challenge to their hegemony.