Yet, "problem," or rather, obvious cultural inequity persists.
The simple answer to the why of that persistence is "crazy religious (and/or repressed) people just get in the way of progress." Which begs the question of "what progress," raises questions of the American exceptionalist trend to view all history as a gradual improvement caused by America, and will henceforth be stricken from this record.
Of course, those same crazy religious/repressed people would, if actually running things, still have women out of the proverbial workplace, allow open racial discrimination in restaurants, et cetera. They don't get away with that, but they're allowed to keep stonewalling gay marriage. Why?
Taxes! Or, more fundamentally, a cool,* complex scheme of antilife financial manipulation that entrenches established order (*in the Machiavellian way, of course! You have to appreciate a good scheme!).
(Antilife? Yeah. Why is being against teh gay antilife? Because sexual behavior and deviation of any kind tends toward life, being of the exploratory, chaotic nature of love, lust, pollen on the wind, and so forth. Right now, the typical "anti" meme is, "They can't have kids," which prejudiced asexual divide-to-reproduce cells would've once slurred at the first sexual cells that went off to swap DNA.)
After all that ado, here lieth the real reasons the plan works/is allowed to continue:
1) Offspring control. Marriage, which once blended well with patriarchy, chattel slavery and parental caste authority--and which still has a cozy, non-dependent relationship with all of these--is, in the neoliberal age, a creature of economic contract. Marriage allows two earning adults to invest economically in one another, ensuring that each of their selfish genes can be passed on in the context of a commitment that's painful (actually painful in the modern world, i.e., expensive) to get out of. This serves to reassure cash investors in a biological future, much like a shared investment of venture capital ensures two business partners that the other person has some "skin in the game."
As standardized rules for the rights appurtenant to corporate stockholders facilitate secured trading on international exchanges, standardized marriage offers to all conforming straight people the ability to invest, at a reduced, government-subsidized cost, in a standard, recognized, court-enforced lifelong contract. Widespread social recognition of the contract reduces entry costs for participants. Denying this privilege to non-straight non-cisgenders subtly increases the economic instability of all unions they might create outside of the established more.
2) Retirement planning. The increased cost and instability of non-recognized unions makes it more difficult for LGBT individuals to plan for the future. In the zero-sum game of the neoliberal capitalist economy, an individual contemplating a shared relationship needs economic protection. How?
Like all things human, efficient relationships benefit from task-sharing: "You do the dishes, the laundry, clean the front yard, feed the dog, manage the finances, and I'll go to work." Zero-sum economics makes it profitable for an individual to take advantage of another by riding out the relationship through an expensive education, an economic recession, or the productive life-years, then bailing without returning the investment that the working half made.
Default marriage guards against this with both social consternation (continuing to become now less influential) and economic balancing tools, such as alimony, spousal support, various forms of "marital property," and other protective provisions for screwed partners. By reinforcing the negative consequences of severance, these increase the security of each partner in a shared commitment, and allow for the easier initial investment. Denying this sanction to LGBT unions (as well as larger-than-two unions) increases their investment risk, making such unions, as compared to "traditional" ones, harder for any given individual to contemplate or deal with.
3) Inheritance, Child and Medical Care Planning. Legally-recognized spouses have all sorts of other nice default benefits provided by the tax base as a whole, including the ability to: act in some fashion as a health care agent and receive health information for the other spouse without expensive or complicated paperwork; inherit from a deceased partner without yet-more-expensive paperwork; utilize default court-recognized power to care for a disabled partner (rather than being utterly blanked out of the system), and act for the protection of another's physical or adopted child (formally recognized or informally countenanced, depending on the situation). "Traditionally" (giggle! snort!) married couples also receive astronomical estate and/or inheritance tax benefits (see upcoming "Tax Theft").
So, there it is: yet another complex, convoluted economic scheme designed possibly by knowing elites and possibly by a small combination of the latter and a large concentration of the subconscious antilife in we all.
(Which, all things considered, does not make it as weighty a world concern as the more direct applications of violence to those who don't have the privilege of facing mere economic discrimination.)