If it was just a private matter, no one would be against same sex marriage. But marriage is important in the standards we set for two people, in how they raise their children and how they pass their family's values onto the next generation. And of course marriage relates to religious freedom and the notion marriage properly understood is the union of a man and a woman before God. The same sex activists do not just want Adam and Steve to marry. They want to end marriage and the family as we've always known it and in the process to do away with the values that its stood for thousands of years. So no - marriage is not just about what two people do or don't do together in the bedroom since their conduct reflects on God and on society itself. The Left would like to make it about sex; the argument goes a lot further than the mere exchange of bodily fluids between a couple.
"Show me just what Mohammed brought that was new, and there you will find things only evil and inhuman, such as his command to spread by the sword the faith he preached." - Manuel II Palelologus