The United States Catholic Conference prepared an excellent Q&A on this subject. I am not trying to re-invent the wheel.
Nonetheless, I would like to tackle the problem from the point-of-view of: What should good people do about this?
So here are some indisputable answers to some crucial questions. (I would be very glad for your comments and additions.)
Vademecum on the right to “same-sex Marriage”
All good Christians must love everyone, and all people are bound to be just. No one has a right to impede the legitimate freedoms of another without good reason. Christians are bound to will the good of others and to do everything possible to help other people get to heaven.
2. Is there an individual right to marriage?
No one has an absolute right to marriage, because it is impossible to marry without a consenting partner. The freedom to marry is NOT, therefore, an individual right. An unmarried man and an unmarried woman–who are not related–are free to marry.
3. In order for all people to be truly free, must we permit anyone who is not married to marry anyone he or she wants to marry, regardless of sex?
Entering into marriage involves a renunciation of freedom. Married people are not free to marry, and they have obligations to their spouse and children. The vows of marriage explicitly renounce freedoms; in other words, they impose duties.
4. Is sex good or evil?
The conjugal union of husband and wife is beautiful, albeit fraught with pitfalls because of human weakness.
Sodomy is inherently ugly. Sodomy is itself a pitfall for people suffering with same-sex attraction.
Sex outside of marriage is selfish. It is not an option for good people.
5. Who has the authority to make laws about marriage, and where does the authority come from?
Civil laws have binding force insofar as they harmonize with the law of God. The state, which enacts and enforces civil laws, arises because of marriages and families.
In other words, marriage is an institution more fundamental than the state. The state has no prerogative to govern marriage. The Church alone has the prerogative to do so.
The Church may concede to the state some practical aspects of marriage law. But no authority can change the constitution of marriage, because marriage is marriage because of the way God made things.
6. Why can’t a man marry a man or a woman marry a woman?
A couple is not married until the marriage is consummated. Acts of sodomy cannot consummate marriage.
7. What is wrong with a man attempting–even though it is futile–to marry a man or a woman attempting to marry a woman?
Such a ceremony would make a mockery of a beautiful and sacred thing. The marriage of baptized Christians is a sacrament of the love of Christ for His Church. An attempted ‘gay marriage’ is therefore a sacrilege, an injustice to all married people, and a crass charade unworthy of any self-respecting civilized society.
…Now, because discerning minds recognize that confusion about marriage has arisen because of two widespread evils, here is a short appendix:
Wedding vows include promises for life. The commitment of marriage terminates only with death, as the vows themselves say.
Bad circumstances can arise which require spouses to separate–even for indefinite periods of time–but divorce is impossible.
9. Can people have babies in any way other than the old-fashioned way?
For a child to be conceived in any way other than through sex between husband and wife is unjust to the child. Everyone has a right to be conceived in his or her mother’s womb, as the result of his parents’ loving embrace. In disputed cases, the rights of children always trump. In vitro fertilization is therefore unjust, and all good people must oppose it.