Scripture never "defines" marriage as only between one man and one woman. Rather, it repeatedly refers to examples of marriages that could be used to define it much differently. Yes, these same folks choose to only consider the story of Adam and Eve's creation for their logic and "Biblical definition of marriage" because it fits their worldview and the way they want marriage to be defined. However, using this same foolish logic, it could just as well be argued that incestuous relationships are "God's will." After all, God only made "one man and one woman." How do you think Adam and Eve's family were fruitful and multiplied? The Scripture also repeatedly says that God not only condoned but blessed the marriages of men with multiple wives, including a whole bunch of concubines to go with it. I guess that means that it is "God's will" that we do the same and we should be advocating polygamy? This argument would be just as ridiculous to so-called "traditional marriage" advocates as their argument is to marriage equality folks. The ridiculous lengths that this line of reasoning can go are limitless. Point being, stop taking select verses and trying to define something as "God's will" while ignoring the rest of Scripture.