Well, if your “prior to 2001” includes only the last 1000 years or so. Because if you go back further, like to ancient Rome or Egypt, there were* legal gay marriages.
I definitely think it is an age issue, with the cutoff point being around age 30. Those of us older than that grew up in a time when the common insult on the playground was to be called a “faggot.” Teachers and parents didn’t promote alternative lifestyles and if someone did come out it wasn’t met with approval, but with jokes and eye rolls. Whether we were affirmatively taught it or not, we were at least implicitly taught that being gay was not normal and not worth recognition by the public as a legitimate lifestyle or orientation.
Contrast that with today’s children. One of my friends’ kids is a senior in high school. A classmate decide to announce that he was gay and was lauded by the principal over the loudspeaker. It is seen as a sign of courage now. When I was in school, the kid would have taken an ass beating before lunch and the principal would have had a conference with his parents about “fixing” him.
When you grow up in a world where gay people are considered absolutely no different than straight people, it’s not hard to realize why these young people overwhelmingly support gay marriage.
Conservatives generally believe that the nuclear family is best, and older people grew up where that was the norm and a deviation from it was the exception. Kids today grow up where the nuclear family is the exception and blended or single families are the norm. If you say that the nuclear family is best, young kids look at you like you landed from 1950. Sometimes I feel like I am in the twilight zone.
It’s simply not true that there has been a sudden shift recently. I’ve seen a comprehensive study of attitudes towards homosexuality in the U.S. Choose any survey question which asks about tolerance towards homosexuality, like one of the following questions:
Do you think that homosexuality should be illegal?
Do you think that homosexuals are mentally ill?
Do you think that homosexuals should be expelled from your church?
Would you disown a child who is homosexual?
Would it bother you to discover a neighbor is homosexual?
Would it bother you to discover that a coworker is homosexual?
Would it bother you to discover a good friend is homosexual?
Do you think that homosexuals should be prevented from holding certain jobs or positions (like school teacher or boy scout leader)?
Do you think that it should be illegal to discriminate against homosexuals in hiring?
Do you think that homosexuals should be allowed to enter into civil unions?
Do you think that homosexuals should be allowed to marry?
Do you think that homosexuals should be allowed to marry in your church?
Chose any region of the U.S. Then the percentage of people in that region who give an answer on that question indicating a greater toleration has been consistently going upwards for the past few decades by about one or two percentage points per years. (Consistently going upwards doesn’t mean that every single poll result goes upward in every single region every single year. If you divide the U.S. into enough regions and ask enough questions, the answer on some questions will move the other way in some years just because your poll is only taken on a sample and there’s always a certain amount of randomness.) Attitudes never really change by huge amounts. Perhaps, as has been speculated above, the real change is that the less tolerant people die and the younger people are more tolerant.