We have no obligation to foist our idea of what we consider to be the greatest good on someone else. Assuming we are dealing with adults, he or she will almost certainly resent your meddling, as well he or she should. It's related to what Warren and Brandeis, in their article, "The Right to Privacy" called "the right to be let alone". You have neither the right nor the duty to be a busybody.

So, let me repeat my question: What do you see are the duties of gays? Specifically, what duties do they have that I, as a heterosexual, do not share with them? If the answer is "none", then they have just as much right to marry as I have. I am speaking of civil marriage, of course.