This view has some intuitive plausibility to me, and carries with it the suggestion that while one or more factors might be absent, the ideal marriage contains all of them.
This might be true. But the argument that they continue from this point is based on the assumption that the ideal marriage is obtainable. Ok, a homosexual marriage may not be the absolute ideal marriage because it cant produce biological offspring (yet)... to show that this makes it worse than other marriages, theyll need to present an example of a marriage (that has actually existed in real life) that is/was flawlessly perfect in every way.
Saying "this marriage has one flaw" does nothing to condemn it unless you can demonstrate a marriage that doesnt have even one flaw.
From there, it goes on to the discussion about the parts that are being "removed" from marriage. But it doesnt attempt to justify the jump from "this is no longer only
the domain of marriage" to "this is no longer the domain of marriage". Just because people have sex outside of marriage doesnt mean that sex after you are married cant have a meaningful significance.
To use their language, marriage does not provide the only
venue for legitimate sexual relations but that doesnt mean marriage doesnt provide a venue for legitimate sexual relations. The same applies for cohabitation. And, for that matter, for reproduction: Theyve missed the obvious - their logic regarding sex also already applies to reproduction. Many people already have children outside of marriage, in exactly the same way that they already have sex outside of marriage. If sex is "no longer the domain" of marriage because it commonly happens outside of marriage, then neither is reproduction, even without
considering homosexual relationships.
All theyve really managed to do is write an essay that points out to people some of the many ways that marriage has problems and doesnt always work. None of which are related to gay marriage.