Atman is right. It's all about attacking institutions and social norms.

The Left contradict themselves all the time.
They don't want to make sense, they want to cripple the "system".

Marriage is a prison for heterosexual women, liberating for lesbians.
Support gay rights, also support homophobic Muslims.
Condemn killing mass murderers, support killing innocent babies.
Deny race exists, then support positively discriminating on the basis of the non-existent "race" of "minorities".

Nonsense. Lunacy. Attack, attack, attack.
I gave up trying to "understand" these people long ago.

Yuyutsu, agreed.
Marriage should be a private affair for both the couple and any businesses who perform related services.
However, there'd still be a problem due to anti-discrimination laws.
These must be abolished.

People should be free to support or reject gays or any other "minority".
Dissent should not be a crime.

Given that gay couples currently have all the financial benefits of de facto or married couples at law, I question why they also want to re-define marriage. Marriage is a Judeo-Christian idea which finds its institution in the heartland of those two religions, namely, the Bible (Genesis). Why would a gay couple want an institutional name which is sourced in a document that opposes gay practise (again, the Bible).
My gay friends live in de facto relationships and have no intention of marrying; it's "my" thing, they tell me, not "their" thing.

TAC "Marriage is a Judeo-Christian idea"

Fraid not.

Marriage has existed in all cultures all over the world, including the pre-Christian European world.

What is universal is its heterosexuality.

There may have been acceptance of certain same-sex "unions" or "bonds", but they did not have the same legal recognition as "marriage".

Even men who shared such "bonds" were still expected to "marry" a woman.

We should tolerate informal "unions" and sexual liberty, but there's no justification for redefining *legal* "marriage", especially as you say, due to de facto recognition becoming the norm anyway.