As we grind towards a SCOTUS decision on same sex marriage, various socially conservative folks and organizations are suffering from an epidemic of pants-wetting and chest-beating.
In introduction, I'd point to the ever instructive tale "And Then There Were None" authored by Eric Frank Russell in 1951 that introduced the term MYOB (Mind Your Own Business) to the language and, to a significant extent, the philosophical landscape.
As with many things, MYOB is a multi-edged sword, as I'll explore later. Hang on tight...this may be a rough ride.
Until quite recently (on a historic scale), government more or less ignored the notion of who got married to whom. After the Civil War a variety of anti-miscgenation (no interracial marriage) statutes popped up, and in the early twentieth century heyday of eugenics the whole marriage license thing (i.e., "folks are too stupid to decide who to marry on their own! Government MUST STEP IN!") popped up.
However, beyond ensuring that participants in a marriage are competent and consenting adults entering into a civil contract of their own free will government has no legitimate role in deciding who can and cannot get married and its sole legitimate service is as a civil registrar and default adjudicator of disputed dissolutions.
Civil marriage, essentially a rather convoluted contract between two competent and consenting adults1, is a legal construct defining the obligations, privileges and responsibilities of those engaged in it (who gets the kids, what happens to property, survivorship, tax and legal rights, etc) and is somehow recorded in an official manner.
Some argue that even the default agreements defined by civil marriage should be dropped in favor of individually negotiated contracts of union, thus further lessening state involvement. See Alabama for recent efforts in this area.
The ethical argument is that civil marriage should be available to all competent and consenting adults, in such configurations as such adults believe will ensure their happiness - with neither subsidy nor penalty imposed by the state.
Religious marriage is an entirely different kettle of fish - a private institution whose activities, within exceptionally broad parameters, are protected by the provisions of the First Amendment. When little matters (say, human sacrifice or the use of peyote as sacraments) do come up for legal debate, they are held up against the harshest form of constitutional examination - "strict analysis" wherein the state actor (usually the folks saying "No. Human sacrifice is NOT ok." or "Peyote is bad, m'kay?") have to sell the judges (or SCOTUS) on the notion that not only is there some kind of compelling state interest, but that the state solution to that "interest" is the least possible infringement on the right in question (here, freedom of religion).
Religious marriage can be considered either a second layer on the wedding cake, or a second sheet cake set parallel on the table to the first. Churches have performed marriages not recognized by the state, Churches have performed marriages not recognized by other churches, and the states have sanctioned marriages not recognized by various churches.
All well and good.
Each faith, within those very broad limits, gets to do its own marriage thing - but that marriage thing, by itself, bears no legal weight. This is also all well and good. Just because a particular faith doesn't like it if a Jew and a Catholic get hitched, or two muslim dudes (or dudettes) tie the knot, or when a Wiccan and a Buddhist of indeterminate gender get married to each other - doesn't mean that those things shouldn't happen - just that a particular faith cannot be compelled to honor or celebrate such unions.
Under such a relatively minor shift from our historic paradigm, everyone - more or less - wins.
Also under the shield of First Amendment protections is the right of people to make statements either celebrating or denigrating either a specific marriage ("YAY!" v. "Jason and Maria - the syphilitic leper marries the sociopathic nymphomaniac, we can but hope they do not reproduce") or a group of marriages as an entire class ("Same Sex Marriage is Bad!", "Inter-racial Marriage is bad," or even "Marriage as a broad general concept is bad!") - and then have those who disagree vigorously respond with criticism and unkind words.
II. You MYOB, too.
MYOB is a game everyone can play. Just be cause you marry someone that - for religious, practical or many other possible reasons - I consider exceptionally bad for either you or for society (or both), doesn't mean I get to do anything more than kvetch about it (and I should probably hold my tongue and let you discover your errors on your own). As long as everybody playing the marriage game is consenting, competent and adult - it's none of my business. (Note that offspring and policy regarding same should be considered a separate argument).
III. Not everyone does it your way, nor should they have to...
Look. There are a bunch of different ways to do the marriage/family thing out there. So long as you start with competent and consenting adults - from a "does the .gov need to get involved" point of view, as a default you and yours should be allowed to go to hell in your own special and personalized way.
IV. Legal thoughts from a lay person
1) Yes. There are exceptions, with some states and nations permitting substantially sub-adult marriages. There are fights about "what's an adult?" and "what does competent mean???" and the ever popular "what's consent?" You'd think all these questions would be well settled, but no - even in the United States there are significantly different definitions for each of these terms.
2) Isn't marriage, and regulation of it a state level issue? Yes, yes it is. But two factors come into play - "Full faith and credit" and the broader requirement that states act within the bounds of the U.S. and their state constitutions. And off we go to Federal court....
What would you propose?
Get the state entirely out of the marriage business and limit their role to that of licensing and regulating private contract registrars (not unlike domain registrars), and beyond that dealing with the bad actors inherent to any situations where you have large numbers of humans interacting.