I will try to answer your questions, and hopefully not get myself embroiled in another religious combat zone. Though I am probably already in the soup for defending traditional marriage laws here. Oy!
Since I am in the U.S. and here we have more religious organizations than you can count and a constitution that protects their right to exist, and I choose to live in this country, I would say that no, one religion could not define marriage. Though some of them do, and do not recognize marriage from outside the Church as being valid, those marriages from outside are not affected by the Churches view unless the people want to enter into communion in those particular churches that feel this way. They cannot cross over, in other words.
I already believe that marriage is defined separately from one religious organization to another, and from religion to government, or civil definition, there are differences. But there is a basic traditional idea that those of us in Christian and Jewish marriages have that is pretty much across the board, and has been for such a long time that it is unsettling to think that there are those who would want to change its meaning.
Civil Unions would obviously be defined by the state governments in which they fall under. Not under church law.
The question about tax law being used to enforce social policies
I need clarification on this question, please. If you mean should the government use our tax monies to blackmail us into complying with their whims, I say NO! But I still want to know what you were asking before I expound on that subject.
Since I am not Islamic I do not believe that polygamy in Islamic marriage takes away from the dignity of the name of marriage as defined from the Judeo/Christian Tradition that I am familiar with. Remember, I think that all religious organizations have their own traditions as stated in a paragraph above. And since this is my thinking about the name of marriage this is why I do not have a problem with same sex couples seeking civil unions. Marriage ceremonies are not going to happen for them in the Roman Catholic Faith or the Orthodox Christian Faith, or the Orthodox Jewish Faith.
As for your statement that I want all marriages made into civil unions, they already are in the sense that couples must obtain marriage licenses from their home states in order to get married even if they are to marry in the Church which doesnt care about the civil part of the marriage except that we must render to Caesar that which is Caesars
the law of the land has to be followed, and since this is the case, the legal rights, or benefits, fall into play already. If we didnt have to get licensed to marry, say, in the Orthodox Church, then of course the state wouldnt know about it and we too would have trouble with our taxes and such.
I hope this made some sense, and I didnt leave anything out. This is a big sticky wicket and so is kind of hard to cover in a nutshell. Im sure you understand.