Buddhists Don't Even Believe in Marriage
If Buddhists don't get excited about homosexuality, they're even less concerned about marriage. Fact is, they don't believe in marriage at all, at least as a sacrament. It's a civil union, a contract between two people. Monks don't officiate at weddings. They do bless weddings, but monks will bless anything including bikes and beer bar grand openings.
Over here, we're all tied up about the difference between marriage and civil unions. Many would let gays have all their civil rights, so long as they don't call it marriage, because marriage is "sacred."
For Buddhists, everything and everyone is sacred already. Including gay people and their love. And a commitment to love and fidelity between two people is just a civil union, but that's plenty.
We're not going to see gay marriage in Thailand or Tibet any time soon. Most Buddhists live in countries that are very conservative socially, and social norms are always stronger than religion. But no matter, they don't have sacramental marriage for straight people either.