Many who believe marriage is between a man and a woman are not bigots who hate gay people. If you are pro-traditional marriage and gay marriage is legal, you still get to think homosexuality is wrong and not engage in that behavior. None of us has the luxury of only accepting the marriages we approve of.
If the reality of all marriages were visible, we'd discover we've always been surrounded by marriages we don't condone. If judged by the Bible, there are many legal marriages that wouldn't pass the adultery test. If judged by harm to children, there are a lot of violent legal marriages that wouldn't exist.
Gay marriage will not affect the number of gay people or couples. No straight person will suddenly become gay, nor gay person become straight based on what the courts decide. Each of us has the right to decide for ourselves what is right and wrong and live our lives accordingly, not dismiss all those who choose differently. You don't have to agree to understand and you don't have to approve to accept.