I can't argue that equal treatment for same-sex relationships is a natural human right (I know many people have tried this argument and it gets swatted down back here every time).
All I can say is that I believe it is negative and harmful to society to have the majority pass laws restricting people who are different from them from having the same rights and opportunities. I see no good reason for it. I think it comes from a legacy of fear and intolerance that has been built into religious belief and I see no reason to value protecting the status quo at the expense of doing real harm to real people and allowing the persistence of a cultural belief that people who are gay are inferior, not like us, not worthy of the same legitimacy. I think we are better off as a society if we do away with discrimination based on sexual orientation even though I don't benefit directly.