>I see far more merit in a gay couple actually honoring their marriage vows than in a hetero couple who will be breaking theirs within a year.

The key issue is the respect of the marriage vows and for each party, not the sexual orientation. I do not think there is any evidence that homosexual relationships are any more stable or monogamous than heterosexual ones. People in relationships (regardless of their sexual orientation) are human and I think the percentage of successful relationships is probably very similar across the board. Just because a gay couple has taken vows does not mean that the odds of both parties in the relationship being true to their vows is any more greater in a heterosexual relationship.