Marriage is seen as a religious institution in my country, and most of the countries represented here, it happens in a church/chapel, it is believed to be in the presence of God, a holy union set forth in the bible. Now although marriage is quickly moving away from this 'holy' view, as now you don't have to get married in a church, or by an ordained person, and divorce is seen as far more socially acceptable than, say 50 years ago, there are still many religious people who view marriage as a religious event, to be done in reverence to God and in accordance with the Bible.
Now, it doesn't take a genius to figure that the main religions and most religious people still believe that homosexuality is a sin against God, and thus a gay marriage is an unholy union, ergo gay marriage is be wrong. It's a pretty simple straightforward point of view.