I see this arguement crop up a lot and I feel like this is largely a semantics issue. "Marriage" as a union under some God and "Marriage" as a package of some 1000+ legal contracts hold the same name for historical reasons (dynamics of the Church in European power structure mostly). When gay mariage is brought up I feel like people just see the religious side and ocasionally the tax breaks and fail to completely grasp the issue. What they gay community wants is to have the easy contract that handles the passing of estates, the combination of insurance, citizenship issues, adoption preferences, etc. Currently we lack the infrastructure to affordably deliver "a la carte" packages; it's the same reason everyone gets the same TOS when installing a particular piece of software. It is not particularly realistic to say that a particular group be denied rights because the infrastructure for delivery isn't ideal.
It's only not realistic because everyone uses this exact same argument every time someone brings this up. We could move slowly towards the correct solution by acknowledging how incorrect the current situation is and fixing it little by little.
Why can't I have an easy way to pass on my estate to someone with all the same tax benefits without having to have sexual relations with that person? Why can't I get insurance options that can be extended to those I cohabitate with regardless of the nature of the relationship beyond the fact that we live together. Why are there not citizen affordances for other relationships such as extending rights to siblings as well? I'm talking exactly about all those same rights the gay community wants. Just like there is no reason many of those rights should be restricted to straight people, there is also no reason that many of those rights should be restricted to two people in a long-term sexual relationship. There's also no reason why we shouldn't be able to pick and choose which rights and obligations we want to opt into or which rights we may want to share with person A and which rights we may want to share with person B.
There is absolutely no reason in this day and age that we can't switch over to a la carte packages over 10-20 years. All you need to do is start offering those options on each of those rights individually and to let all the rights for married couples expire and for those that don't expire, you can work on sunsetting them once a suitable a la carte solution is available.
