Marriage should be a right between any two people who are consenting age and are actually in love. The bible goes against gay marriage, sure, but the bible is also said to be against those who have sex outside of wedlock. In this, not only by saying you are a sinner for being gay and wanting to marry someone of your own gender, but you also call them a sinner for not being able to wed, so having sex out of wedlock.
Other than this, it's wrong to base a book to judge for your entire life, your entire civilization. As a book of morals, don't steal, don't kill, it works fine, but with so many people who are openly gay, bi, whatever, there's no excuse to try and judge them.