Gay Marriage though would be equivalent to an alcoholic who wants to be a bartender. Besides the fact that the marriage could never be valid, Catholics are opposed to gay marriage because it would create the constant “occasion” for sin, which needs to be avoided by everyone at all times.
This is not at all the reason why the Church is against same-sex marriage. The real reason is rooted in the Natural Law.
In the marriage covenant we are to come together “as one flesh”. The marriage covenant is therefore consummated by sexual intercourse. In human physiology, the marital act is the only act which actually requires a male and female, to accomplish. For the purposes of procreation, at the moment of consummation a husband and wife are indeed “one flesh”. Unlike every other bodily function (respiration, digestion, etc.), procreation
requires two persons to accomplish.
Even if the act is sterile (menopause, pregnancy, illness, lactation), the parties are still engaging in the physiological act of intercourse and of coming together as one flesh. In the case of sterile (for legitimate reasons) intercourse, it still carries out the unitive function and the two parties are still engaging in a natural physiological process even though that process is not naturally fertile at that moment.
There is no homosexual equivalent. All human physiological processes are designed to be either life-sustaining (breathing, drinking, eating), or life-creating (reproduction), and all are necessary to maintain the human race. The life-sustaining processes are self-sustaining (only need yourself), and the life-creating process requires two people.
Homosexual sex is neither life-creating nor life-sustaining. It’s only function is pleasure for those who find it pleasurable. A same-sex couple cannot “consummate” their relationship. It is a biological impossibility. If all humans only engaged in homosexual sex, the human race would die out. Same-sex activity is neither life-sustaining, nor life-creating. It serves an entirely hedonistic purpose, just as contracepted heterosexual sex serves an entirely hedonistic purpose.
Therefore a same-sex relationship can never be a marriage. At best it can be seen as an imitation of marriage, or an analogue to marriage.
But it is not, cannot and never can be a marriage, because two homosexuals cannot come together as “one flesh”. They can come into intimate physical contact, but it will never be as “one flesh”.
This also opens up a legal can of worms. In both the Church and in society, a marriage that has not been consummated can be declared null. Non-consummation is easy to define: it means sexual intercourse has not taken place.
How do you define non-consummation for homosexuals? Simply not having sex? What kind of sex? Homosexual activity can be a wide array of practices. One party can claim the relationship was consummated and the other claim it wasn’t based simply on sexual preference. It is clearly nonsensical and potentially a legal quagmire.