I did not say that cannot be the intent. In fact, the Catholic goal is to preserve the procreative and the unitive aspect.
So sex for the purposes of intimacy, not procreation, is also allowed?
If you're using condoms to prevent the spread of STDs, especially if you have the STD, you shouldn't be having that sex in the first place.
That's like tackling drug abuse by saying "Don't use drugs". Evidently, a stern telling off is insufficient to prevent STD transmission. More than that, a person might not know they have an STD. A person might know they have one and be willing to use a condom to have sex - but, in the absence of condoms, has sex unprotected. A person might be driven into prostitution to survive (condoms have
demonstrably helped stifle STD transmission in this case).
Making sure condoms are in good supply is better than telling people off. They
will have sex, regardless of how deeply we furrow our brows, so we may as well ensure they have
safe sex, rather than
unsafe sex.
Abstinence, whether permanent or temporary, does not count as a 'purposeful act' because it is the absence of the sexual act.
Inaction is still a concious choice you make. You
could be out having oodles of babies, but you
choose not to.
Pragmatically, people who pledge abstinence
are as likely as everyone else to engage in sex - but
are less likely to protect themselves from diseases. We can quibble over what to call it, but abstinence isn't effective at preventing disease, not in the extreme cases I mentioned earlier, nor in ideal cases I gave just now.
So, presumably, you don't approve, going by your claim that: "
the act's potential to result in life must not be altered by man so far as the intent of said alteration involves the desire to not have any children". By marrying an infertile woman, as opposed to a fertile woman, the sexual act's potential to result in life has been altered.
Why the dichotomy? It is important to maximize offspring WITH thought to the risks while engaging in a lifelong commitment, which is the goal of NFP.
That as may be, but there are far more effective ways of accomplishing this, vis a vis, condoms. Suppose a woman falls in love with a man who was barn with HIV/AIDS - should she abstain from sex forever more, thereby diminishing her odds of having buckets of babies, or should she engage in sex for the purposes of procreation even though her life is at risk? What does the Church deem more important - quantity, or quality, of life?