By far the most common historical marriage was one man, and whatever number of women he could acquire by force or negotiation.
The idea of "one man, one woman, who monogamously love each other" has only become common in the last couple centuries, and in some countries still isn't. THAT is the radical redefinition of marriage. If you can support such a revolutionary change of thinking in what "marriage" is, it's mind-boggling that you can't also support the comparatively minuscule edit that same sex-couples are OK too. It just doesn't stand to any kind of logical scrutiny.
Harry, why are you bantering with this person? Like you said, being anti-gay marriage is no different than being pro-Jim Crow back in the day. They are not worth talking to on this subject.
Democrat to Democrat, I think our party has made up our mind on this one. I'm not normally one to say "fall in line", as different factions within a party are essential. However, I make an exception for this issue. This is a Civil Rights issue and in my mind, to be a Democrat you must absolutely stand for equal rights.