In an article at Huffington Post, Malcolm Clemens Young (a Reverend Doctor in the Episcopal Church) attempts to answer What Do Christians Supporting Gay Marriage Say About the Bible?.
First they must dismiss clear statements in the Old Testament. Since it has a lot of rules no longer applicable we can safely assume this one is not applicable today. He raises the issue "How do we know which commandments we should guard with our lives and which ones make little sense in our context?" As far as I could tell, he never actually answers it. But since we ignore the rule of plowing an ox and ass together, surely we can ignore women having sex with women and men having sex with men.
Now, having driven any right to speak to the issue into the New Testament, they must The first order of business is to highlight the fact that Jesus did not directly condemn the practice. Young writes, "It certainly is not prohibited in the gospels," and "Jesus does not explicitly mention it." This ignores the fact that Young and many other same-sex supporters find many things they think are wrong that are not "explicitly" addressed in the four gospels. It also, to readers in the know, implies that the rest of the New Testament is not as authoritative as the gospels. While Jesus may not have explicitly condemned homosexuality, He nevertheless condemned it in His positive teaching on marriage. Jesus condemned all sexual relations outside of marraige, which He based on the one man-one woman model established by God and recorded in Genesis 1:27.
Young characterizes the apostle Paul writing "briefly but also cryptically" on the subject -- so that he has "no idea what kind of relationship he is talking about." Further, "this issue certainly is not the heart of his message." Heart of his message or not, it was part of Paul's message -- a part that has not been unclear for 2000 years until some people called Christians decided they want to do something different.
What do Christians supporting "Gay Marriage" say about the Bible? They sometimes "say" things that obfuscate, confuse and emasculate the Bible of its authority. What do they "do"? Mostly they do what most others do -- take the parts of the Bible that they like and leave off the rest. Right or wrong, they're not doing something that American Christians haven't been teaching themselves and others to do for years & years.