I would suggest that we argue what is essential to marriage, union and procreation. It doesn’t really matter how you argue against gay marriage, if you can’t convince people that procreation is essential to marriage than you have no hope of convincing people that gay marriage is ok.
I mean think about it, what is the difference between opposite sex marriage and same sex marriage?
In a traditional marriage, two people who feel very close to each other decide to enter into a long-term relationship.
In a same-sex marriage, two people who feel very close to each other decide to enter into a long-term relationship.
what is the difference? Pretty simple a traditional couple can reproduce, a same-sex couple cannot. That really is all it comes down too. If you can’t argue that reproduction is essential to marriage, just as the Church teaches, we lost the argument before it even starts.
Personally I believe the wide acceptance of contraception is one of the main causes of this wider acceptance of same-sex marriage. IF sexual relations in a marriage, doesn’t need to be open to children, what makes it different than any other sexual relationship?
In all people there is the sexual drive, it is a part of who we are naturally, and it is why we have sexual relations, a part of our nature is continuing the species, but of course, we are more than animals so we don’t enter into sexual relationships simply to continue the species, we also enter into them because they allow us to deep intimate relationships that also help each other lead to salvation.
Anyway after this long explanation let me simply say, to support traditional marriage we must prove procreation is essential to marriage.
This is why impotence is an impediment if, now infertility is another issue and I’m no expert on that.