It isn't a matter of common sense. There is nothing "common" about "sense". Sense is always a matter of personal opinion; there is no absolute truth. And that's really the crux of the problem, because the reason for the controversy is power; specifically one group's effort to exert power over another, in the interest of dictating for everyone what a personal committment between two people should be allowed to be. Their interest is in having the secular definition of the institution match their own religious definition, rather than recognizing that reasonable people disagree about matters of belief, such as this. They resist the idea of granting people, other than themselves, the right to live in accordance with their own beliefs on matters such as this.