There are various objections to expanding the conventional, up-tight, as-God-intended “one man, one woman” notion of marriage but by far the least plainly bigoted ones I am aware of are the bureaucratic ones.
To be blunt, the systems aren’t set up to handle it. The paper forms have a space for the husband’s name and a space for the wife’s name. Married people carefully enter their details in block capitals and post the forms off to depressed paper-pushers who then type that information into software front-ends whose forms are laid out and named in precisely the same fashion. And then they hit “submit” and the information is filed away electronically in databases which simply keel over or belch integrity errors when presented with something so profound as a man and another man who love each other enough to want to file joint tax returns.
Speaking as a computery-type person, altering the paper forms is not my department. It’s probably expensive and there are probably millions of existing incorrect forms which would need returning or recycling or burning instead of using. Or maybe it’s simple. I don’t know. The real question from my perspective is how you store a marriage in a computer.
Altering your database schema to accommodate gay marriage can be easy or difficult depending on how smart you were when you originally set up your system to accommodate heterosexuality only.