I actually think it's two pairs of shoes you're discussing here. One is the marriage as seen by the state/government...something that is really needed to give the gay couple the rights any heterosexual couple has, like giving the partner the official right to visit and get information when the other partner is in hospital for example, or give the security needed (like inheriting house, money, etc) when the partner dies)...you get the idea. The other end of the story is the marriage in the face of the church and God.

While the former has been legalized in a lot of countries, the latter is a lot more tricky since the church still refuses to give their blessing to homosexual couples.