You are correct in that it is not the best argument against homosexual unions, IMHO.
However, it is still a valid one.
Yes. And has been already pointed out, their union is still ordered towards procreation, while homosexual male sex is not. (There cannot even be any argument that homosexual female sex is even remotely ordered towards procreation. There is no sexual joining possible.)
I believe we’ve already had this conversation, rossum, on a thread of a similar vein.
We’ve already talked about why an infertile couple (say, a couple in their 60’s) can have a valid, sacramental marriage, while a homosexual couple cannot.
Here’s what the difference is between an infertile couple whose union is still ordered towards procreation (although who may never conceive) vs a homosexual sterile couple who will never conceive. (They can’t even engage in intercourse, actually.)
Take this analogy, borrowed from
Bad Catholic.
Imagine a senior citizens baseball team that shows up to play the 2011 World Series champs, the St. Louis Cardinals. They have absolutely no chance of achieving the proper end of baseball (gaining more runs than the other team), but so long as they play according to the rules** their play is still ordered toward its proper end.**
But in the case of homosexual unions, the play itself is changed. It would be like showing up at the game without bats, wanting to play with their backs to each other, so that even if they have the
intent of winning a baseball game, what they are doing can’t possibly be ordered toward that end because** they are, quite simply, no longer playing baseball. **