Serryah, I agree with your fiancée, I also think it is only a matter of time. Any civil rights movement takes a long time to succeed. How long ago was Rosa Parks? It took a long time for blacks to achieve equality in the eyes of the law (they still have not achieved equality in the eyes of the society).
You should not judge the success of the movement in terms of weeks or months, but in terms of years or even decades. Where was gay marriage ten years ago? It was nowhere; even Canada did not recognize gay marriage. Now several countries in the world recognize it, some states in USA recognize it.
Indeed, there is strong support in younger generation for gay marriage; they support gay marriage by a big majority. As the older generation dies out, more and more states will legalize gay marriage.
There has already been a substantial movement in public acceptance of gay marriage. Until recently, 65% of Americans were opposed to gay marriage, recent polls put it at 55%. So currently the opposition stands at 55:45. A movement of just two or three points (a relatively small movement) and it will be 50:50.
No civil rights are ever obtained in a straight line; it is always three steps forward, two steps back. However, the long term trend is clear, gay marriage is here to stay. Eventually it will be legal all over USA (though Bible Belt states may not legalize it for perhaps another 50 or 100 years).