I'm not looking for a debate, but I'll try my best to answer your question. In regards to John Kerry, he lost... primarily... because of his inconsistencies. He was all over the place on the issues. He was recorded as constantly changing his stance on the War on Terror and the Republicans did a good job at exposing this weakness. He is a Catholic, but is pro-choice... that didn't sit well with a lot of Catholics and Protestants. He isn't for gar marriage, however. He is actually for civil unions for gay couples, which I don't have a problem with. Much of why gay marriage is adamantly opposed by the majority of Americans is because this nation, since the day of its founding, has been mostly populated by people of faith. Keep in mind, people of all faiths... Christians (Protestants, Catholics, and Mormons), Jews, Muslims, Buddhists, and Hindus. You take those people away and you have a small percentage of people left. Most of the people of those faiths tend to be against gay marriage and abortion as well. Most Americans don't understand why a child should be denied the right to live if a couple was irresponsible one night. It's different if rape or incest is involved... but the issue is touchy, because as President Ronald Reagan pointed out: "I notice that everyone in favor of abortion has already been born."
Bottom line: I think the primary reason behind the inability to mobilize support for gay marriage and/or gay marriage is because of religion.