Why is the state involved in marriage at all?

[Rev Dark]

No, Morrisey is writing twaddle.

Marriage as run by government has been disintegrating for decades, as
the divorce rate shows, and that has nothing to do with gay
relationships.

Whoa there Hoss.  Not true, not even in the same ballpark.   Logical fallacy.  Drawing a correlation between government 'running' marriage and the divorce rate is pure twaddle.  "You cheated on me, beat me, belittle me in front of others; this marriage as run by government just isn't working for me."

Marriage, as run by the government, is the only contract in this
country that can be broken by one party alone with no adverse
consequences.

Again; outright fucking lie.  There are adverse consequences in regards to the division of property, custody of children and pets, etc.  Common law marriage; if you are the sort of douche-bag who lives with a long time girlfriend then dumps her, leaving joint debts, you are legally obligated to pay those debts and maintain contracts (rent/lease/mortgage) that you entered into.  Kids?  Your responsibility.  You have one, you are on the hook.

We would do much better to require people to create partnership
contracts in the civil context than get marriage licenses for issues
like property sharing, access to family, and so on. If people want to
live together and share their lives to that extent, it’s healthier and
much less confusing later to have those issues expressly spelled out in
an agreement up front, just like any prenuptial agreement today. If two
people don’t want to go that far in formalizing their relationship,
then they shouldn’t be considered married anyway — and shouldn’t get
access to “palimony” and have debates over oral contracts, and so on.
If you don’t get it in writing, it doesn’t exist, in the context of
personal partnerships.

Which the rule of law already covers; it is the extension of that rule of law to cover same-sex couples that is at issue here.  Head so far up Ayn Rand's bottom that he's touching Galt. 'Yes Dagny a 60-40 split on fixed assets is acceptable, but I will be retaining 80% of future revenue on Reardon Metal.'  A system rife for abuse.

Let government define and enforce contract law, not marriage. If we
don’t follow that path, people will shortly become very unhappy about
the eventual government definition.

The government already enforces contract law; with a common set of criteria in regards to what is referred to as 'marriage'; this is about extending those same extant privileges to same sex couples.  Being opposed of course, by a set of douche-bags who favor the 'traditional' marriage of their faith.  (Have you read the Bible, Book of Mormon, Koran? - yeah, they all espouse the 'traditional' marriage.)