With any form of contraception, including sterilitztion, besides abstinence conception can take place. The egg is only viable for 2 days. All other days are infertile. The issue is finding those two days.
I don't have source documents, yet, but here are some quotes for discussion.
East: St. Clement of Alexandria says in A.D. 191, [The Instructor of Children 2:10:91:2], "Because of its divine institution for the propagation of man, the seed is not to be vainly ejaculated, nor is it to be damaged, nor is it to be wasted."
WRH: The seed is something precious, and wasting it is a mortal sin, as we see from God's swift and just execution of Onan. Sterilization (damaging the seed) is impermissible, and so are all types of outercourse; all intercourse must be vaginal.
The saint adds [The Instructor of Children 2:10:95:3], "To have coitus other than to procreate children is to do injury to nature."
WRH: St. Clement teaches that any sexual intercourse that is not open to life is unnatural.
We see here the absolute prohibition on having intercourse during the times when the woman is unable to conceive. When, through the use of the NFP method, it is known to be an infertile time for the woman and the male ejaculates uselessly into the vagina, this is, as the Catholic commentary says, a mortal sin and against the Natural Law,
Unfortunately those who use NFP mostly use it to achieve this state of mortal sin.
I have no idea how the Popes reconcile their modern teaching with patristic teaching. It is more than obvious that these Fathers would prohibit all intercourse during a woman's infertile period.
There is no such thing as "infertile" periods in the woman's cycle. There are only "less fertile". Therefore, NFP is taking advantage of the "less fertile" with full realization that conception can, and sometimes does, take place.
But, in all those forms, you have changed the sex act.
No, it's the same act. It's adultery, for instance, if you sleep with someone else than your spouse, even if you use a condom. With the IUD, the pill, sterilization (the first being immoral, the second questionable, and the third being moral only in limited circumstances IMHO) there is no difference in the sex act itself at all from "unprotected" sex.
I didn't say it was a different act, I said it is "changed".
Adultery is irrelevant and a different sin altogether.
NFP is the same act, just timed during "less" fertile periods.
With the high dose pill, sterilization etc. it is the same act, just making it that much less fertile.
And with a pill, or whatever, you've physically changed the natural state to meet your needs.
Men do that with a viagra pill (btw, I vehemently disagree with Fr. Josiah Trenham on this issue: there is nothing sinful about using viagra in an of itself, and he seems a bit confused in any case over what viagra is for).
I use an inhaler to physically change my natural state to meet my needs, i.e. breathing.
Btw, the pill is often used to make a woman's irregular cycle regular, i.e. the natural state to meet whatever needs she has for that. (Elijahmariah will love this one: Muslim women often use the pill to prevent their periods from coming during Ramadan, as shari'ah forbids them from fasting while menstruating. They can, however, have sex and eat during the night).
Engaging in intercourse during the infertile period rather than the fertile period changes the sex act, as you say. During so when you are trying to conceive versus doing so when you are trying not to conceive, are very different. (In fact, no two sex acts are entirely alike. If they are, that brings up other problems). Intercourse during the infertile period resembles intercourse with a condom more than it resembles intercourse during the fertilfe period trying to conceive.
Not to be indelicate, but to stop comparing apples and oranges (as very few people have ever expressed a fondness for condoms, no matter how much they use them), compare intercourse with a lubricant (or do you think that is not allowed?) and that with a lubricant with a spermicide.