A theory on marriage equality
It's also about legal rights and protections, of course. The ability to adopt, insurance benefits, visitation rights in hospitals/prisons, significant tax breaks, citizenship, etc, etc. But really, the most important point here is that marriage is something two people decide between themselves. The state doesn't have to bless a marriage like a church, all the state does is sign the contract, formalizing the legal arrangement. It is assumed, automatically, that two adults are capable of making that decision for themselves.
No one would ever try to deny the legal right to marry to two folks who just met during a bender. No one would try to deny the legal right to marry a pair of convicted felons. This has never actually been about the romantic durability or the moral integrity of the couples involved - it has always, simply, been a matter of controlling the other. Of reacting to carefully constructed homophobia as part of the ongoing superiority project of heterocisnormativity. People are screaming because they've lost that control - even if they think it's about something else, that's just social brainwashing.
You see, it really comes down to this. The legal right to marry is a sign of competent adulthood, i.e. PERSONHOOD. That has always been what they wanted to deny. Same-sex couples aren't supposed to be real - the reaction to them is about as composed as the reaction to someone who dresses up like a medieval wizard and expects to be taken seriously in day-to-day life (which would be awesome by the way, but that's another point). "You aren't supposed to be doing that, you're engaging in a fantasy that challenges the authority of the world I know." Dehumanization, and the stripping of rights, asserts the order of that reaction.
Now that this right has been secured, some people's grip will loosen. Other people's grip will tighten. Stay vigilant for the wild, flailing throes of the anti-personhood brigade, as we continue to push for full personhood, with all the attendant rights and protections, for all GSM (Gender and Sexuality marginalized) folks.