People automatically assume that if you're not pro-life, you're pro-abortion. I am pro-choice, that does not make me pro-abortion. Although I would personally never have an abortion, I believe a woman should have the right to choose.
Imagine how stupid you are going to look in 40 years. In 40 years, everyone will think that people who protested gay marriage are just as stupid and bias as people who protested inter-racial marriage 40 years ago.