This came from an argument for same sex marriage found here.
In it he brings up four very good questions that many Protestants don't understand about Catholic human sexuality. After I answer his four questions, I'm going to throw in a fifth that most people don't understand, and then I'll wrap up with a why at the end.
1. Why should sex by an infertile couple be considered "open to procreation" when sex with a spermicide isn't?
This is the Abraham and Sarah answer- Sarah was considered infertile when she has Isaac, the start of the whole Judaeo-Christian world. Sex by an infertile couple is considered open to procreation because miracles of this form have been known to happen throughout human history. I'd point out though that while medical science now can duplicate this miracle- it's still only RCC-approved if GOD does it, not MAN.
2. Is Natural Family Planning considered acceptable only because it is less reliable?
No, and in fact, modern NFP is more reliable not less than many competing forms of birth control. Rather, the ideal form of human sexuality for the Roman Catholic Church is to follow St. Paul- and not do it at all (several times in his letters Paul recommends celebacy over marriage, only reluctantly allowing it to "prevent worse sin" and when it becomes clear that Christ's return isn't going to be tomorrow). NFP shows solidarity with celebacy because it is based on abstinence (more on this in question 5 below).
3. (If condoms were less reliable, would they be acceptable?)
Nope, because the problem with condoms isn't their reliability, but rather man's attempt to thwart the will of God in procreation. Remember, the ONLY reason Catholics are allowed sex at all is for procreative purposes, and while this does include the unitive aspect of sex (keeping the parents together in marriage at least long enough to raise the kids), it doesn't include specifically denying God's will.
4. Is there really anything "open to procreation" about a couple who know they are infertile because of physical deformities, age, medical conditions, previous surgery, or any other reason?
Back to Abraham and Sarah, and the answer is YES. Except in certain "previous surgery" cases that is, where the purpose of the surgery was infertility (for the same reason cosmetic or any other elective surgery is also considered ethically questionable by the church).
and then a question brought up here:
5. If homosexuality is bad because it's not open to procreation, why isn't celebacy?
I think I've skipped a logical point here that caused this question to be asked, and that's my fault. Holiness in the Roman Catholic Church is all about denial of self, and submission to God's will- I think that's why some in the 1960s and 1970s explored Zen Buddhism as well. The Church's teaching on human sexuality includes two levels of holiness, and two levels of sin:
1. Celebacy. The ultimate denial of one's sexual orientation, by simply not having any. Denial of self for the will of God.
2. Marriage & NFP. Denial of oneself for the continuation of the species, which is ordained by God in the Book of Genesis.
3. Monogamous but outside of Marriage or Using Birth Control, or these modern extended families built out of serial monogamy, aka divorce- I kind of get the feeling this is considered a Venal Sin, not Mortal. I'd put monogamous homosexual couples in this group- denying the procreative aspect of sex, but as welcome as any other sinner in the church.
4. True depravity- that 1% of fetishists and the like that makes the rest of us gag, whose sexual practices and addictions are so far outside the norm that even the most tolerant of us says, we can't support that.
Burn in hell for all eternity? Just being homosexual and monogamous won't make you do that. Jesus loves you too much to allow that to happen.
And I need to say a word about divorce. If your marriage is sacramental- that is you take seriously your vows and you try your best to place Christ at the center of your marriage and you're a good level 2 heterosexual Catholic as listed above, it doesn't matter where your lusts lead you- you've chosen your path, and NO other options should be available to you. Commitment, not love, is at the center of a good Catholic marriage- because love may come and go, but you've made a commitment to yourself and your God to stick with this family NO MATTER WHAT.