First time getting involved in this thread, because I'm finding the question being asked is the wrong question to ask (my opinion). Is the issue here about "marriage", or about a legal recognition/acceptance of a homosexual couple commitment to each other for life?
A heterosexual marriage (even though legally recognised/accepted), is no longer viewed as a necessity (even by law), for the couple to have certain rights over each other should they decide to go their separate ways some day in the future.
So based on my understanding (and I'm happy to be corrected), should not the question be more about whether one (or the law), ought to view a marriage between a homosexual couple the way it views a heterosexual's marriage, and if no, then why not?