The institution of marriage generally is based on the idea that human society retains some natural inclination to follow the lead of most higher vertebrates to pair up, for mating purposes primarily, but also to create a nuclear family unit from which more complex social structures are supposed to proceed. I can't speak about the process by which the mating of geese is regulated or sanctified within anserine societies, but in Western human cultures (to bypass altogether the question of whether marital pairing is strictly necessary any longer for any purpose at all) it has been regulated by two separate authorities, the church and the state. Miraculously, the institution of marriage has made it to the present day straddling these dual realities without much conflict.
For most public religions, the biological function of marriage is the one favored by God. The web of family and church affiliation is the approved structure by which Christians, Jews and Muslims are encouraged, sometimes required, to live their lives. Revealed by the very nature of reality, the coupling of opposites followed by the mystery of fecundity is an example and extension of the mode of all creation, yang and yin, form and matter. We depart from the man/woman paradigm at risk of estrangement from our religious ties and loss of integration in this world and the next.
Lacking religious influence, marriage falls under the purview of the tribe. This evolves from regulation by clan tradition, through more inclusive governing authorities, all of which could be said to constitute the state. State concerns are different from religious concerns. They are more about ownership and divvying up assets. In our culture, marriage is a form of legal partnership determining issues of joint ownership, taxation, inheritance, liability, custody and so forth. Divorces are not issued on the grounds of love or lack thereof (which isnt a legal concept), but rather of demonstrable breaches like infidelity or abandonment of responsibility. Lawyers engaged in the contested dissolution of marital contracts must have a positive appetite for lunacy. Similar in the tendency to acrimony and the squabble over assets, marriages are dissolved in the same way as other partnerships.
The problem today is that marriage serves a different purpose in each of these two realities. The state doesn't care whether couples love each other, or whether they embrace certain moral or metaphysical beliefs, since these have no effect on the rules of joint taxation. The church doesn't get involved in property settlements or custody disputes unless, as in many Islamic societies, the church is in fact the state. The church might be threatened by the idea of a homosexual marriage, or even an interfaith marriage. The state should have no such taboos, so long as the partnership is legal and the laws of partnership apply.
In this hemisphere it is not mandatory to belong to a church. It is mandatory to belong to the state. Only the state, therefore, can truly be repressive, since its rules are escapable only by giving up citizenship. Presumably we belong to a church because we embrace its beliefs, so if we find ourselves at odds with those beliefs the appropriate alternative is to find a church more to our liking. The choices available to those who disagree with civic laws are to change the laws, accept them under protest or get out and start a new life elsewhere. Either way, it is the prerogative of both institutions to determine their proper rules of membership, to amend them as they see fit, or to refuse to amend them if this would contradict their defining principles as they understand them.
To my thinking there is no reason why the state should not issue a marriage license to virtually any two people who request one. Or, for that matter, any three or a dozen people, since professional partnerships can be of more or less any size. There is no reason why we should not marry our sisters or our dogs and cats or our entire homosexual glee club, as long as the papers are in order and the taxes are paid and everybody gets on the family dental plan. As far as I am personally concerned it is perfectly fine for all consenting parties, married or not, to slather themselves with lubricant and sleep in libidinous, incestuous, transsexual, transracial, transspecies heaps. Given responsible precautions against the spread of disease, such recreational buggering should be part of everyones civil rights.
But do we then have the right to go to our priest, who lives in quite a different world, and demand that the one man-one woman restriction with which scripture is permeated from one end to the other, be amended to allow what is contradictory or abhorrent to Catholic doctrine? In order to agree with you, your priest would cease to be Catholic, just as in order to agree with Ariel Sharon a Palestinian representative must cease to represent Palestinians.
It is the absolute right of any club existing to promote any activity or credo, however odious. Stamp collecting societies should require that their members collect stamps. The Boy Scouts should not have to admit girls. The League of Women Voters should consist of women voters. White supremacist gun clubs should not be forced to admit black members. It is proper to deny citizenship to anti-American terrorists. If Hale-Boppsters are unwilling to wear orange jump suits and drink poisoned Kool-Aid, they should be booted out of the club. Catholics believe that the Pope is the earthly vicar of Christ on earth, and that lacking a new divine revelation amending their backlog of prohibitions on birth control or gay marriages, the rules remain justifiably opposed to these things.
It is perfectly correct for them to do this, or more exactly, it is a denial of who they are to do otherwise. It is not a question of whether traditional religion is in general deluded, or whether the Pope is correct or incorrect in nixing same-sex weddings. We may lament his position, but to excoriate him as an anti-gay monster of some kind is not appropriate. He is just defining the limits of Catholic orthodoxy. He is a theologian. He isn't forcing anybody to join the club.
Gay Catholics wishing to become nuclear families have abundant choices. They can remain in the Church and cohabitate, just as millions of unmarried Catholics already do, with no particular stigma. They can remain Catholics and get married in a civil venue, thus enjoying all the legal benefits of marriage. They can stay put as renegade Catholics in company with the 70% of Catholics who disagree with their infallible Pontiff about death penalty abolition. They can, if the conflict of beliefs is too onerous, either continue to press for doctrinal reform or trade in the stuffy papists for a denomination which recognizes a more liberal Deity.
Marriage, like sex, is too crazy to figure out, let alone to get everybody to agree about. There isn't any real reason for it to be the way it is, or for it to exist at all. Who can find a reasonable starting point to even begin thinking about it? If there is a special someone you would like to climb into a tub of chocolate pudding with and it doesnt seem like you could get arrested for it, cripes, go ahead.
Which of our body parts is it and is it not okay to rub against the body parts of other people, given a hopelessly complex set of permissive conditions including intention, circumstance, cultural environment, relative sex, relative age, marital status, state of sobriety and on and on? What, in its detailed two-page legal/anatomical description, constitutes a human butt for the purposes of a Fort Lauderdale decency law regulating the required body coverage of a string bikini? I doubt whether Jesus cares, and neither do I.