The majority of those who oppose same sex marriage offer civil unions as a substitute, saying that this negates the equality argument. The problem with that solution is that laws are enforced as written. There are literally tens of thousands of situations in civil law and common custom that involve marriage, from those of crucial import down to simple common courtesy, such as this story: Same sex couple, kids denied family rate at pool .
The only way civil unions can be equal is if they are declared legally to be identical to marriage in all circumstances, criminal and civil, and enforced... which would then put us in the untenable position of claiming that a union that is identical to marriage, was called a marriage by the church that married them, and was registered by the same county clerk as a marriage, is not in fact a marriage. Anyone who can do that has a promising career in theology or political science in front of them!