My marriages failed because of ME.
Before I had ever heard of gay marriage.
I probably could have saved at least one of my marriages by doing a lot of things different; by putting forth a lot more effort and sacrifice. But what uninvolved "others" were doing couldn't have saved them. My marriages would have failed regardless of what relationships or marriages other people- most of whom I don't know and will never meet- choose to engage in.
So, to claim you are "defending marriage" without addressing the reason mine failed- probably the same reason the vast majority of marriages that fail do so- is silly.
No one else's relationship status can threaten my relationships, or weaken them in any way, unless I let them. And that would probably mean I am focusing on the wrong thing.