I'm not really well versed in the typical pro and contra arguments, so this made me curious.
The moral argument against homosexual pseudomarriage is that it represents a fundamental misunderstanding about marriage. Marriage is not about a couple's (or a group's) feelings about one another. It's not a way that society shows approval. It is, rather, the formation of the most basic unit _of_ society (I'd argue that the individual is not really a unit of society), and is the means by which children are produced and brought up.
(And yes, there's plenty of heterosexual pseudomarriage too)
But, given that these are all moral arguments, my preference is for the State to absent itself entirely rather for it to take a side and do further damage to an almost destroyed institution.
In fact, if marriage is 'almost destroyed' as you posit, then we need more, new styles of family if kids are to have a chance to grow up fed and clothed and loved enough.
How does adoption in general work? For example it can happen through "natural causes" that a child has only one dad and no mother. It seems likely that having two dads would be better than just one. But if somebody wants to adopt, they might have to be "better than average"? Like would a single dad have a good chance of adopting? Or does some office consider the likelihood of a child's wellbeing in a family - so presumably they would try to find two parents, not just one, which is discriminatory against singles? And then if the "adoption office" (or whoever is responsible) has the choice between a heterosexual and a homosexual couple, things become difficult?
My cousin has 4 kids, and is married to the love of her life Lisa. Their kids are certifiably the kindest, most considerate kids I know. There's been no psychological damage, at least not at their hands. Ignorant outsiders may say mean things but to blame that on these excellent parents would be twisted logic.
Anywho, the decision about adoption is so far down the road from marriage as to be a red herring.
But I think you are wrong about adoption. I think it's one of the main concerns opponents of gay marriage have. What other rights would people be concerned about? The other one I can think of is the right to bring your spouse into your country.
Are people going to the barricades because they don't want gays to be able to visit their partners in hospital? I rather doubt it. Adoption is one of the big issues.
I think, because the argument is thrown up semi-randomly - See! They might adopt! That would be awful! Just another red herring, saying anything at all that might convince a voter to strike down the right to marry.
People who continue to make this argument are ignoring a very well-established conclusion in the social science research.