Natural Family Planning (NFP)
NFP is quite simple. Couples using NFP to avoid pregnancy abstain from intercourse and genital contact during the fertile phase of the woman's cycle. No drugs, devices, or surgical procedures are used to avoid pregnancy. 
The encyclical letter Humanae vitae issued by Pope Paul VI in 1968 clarified it for the catholics.
Consequently, it is a serious error to think that a whole married life of otherwise normal relations can justify sexual intercourse which is deliberately contraceptive and so intrinsically wrong. In plain English, it means, contraceptives like condoms, pills, IUDs etc are all morally wrong. Every sexual act should be open to procreation. By using contraception, you are not open to making babies and that is a sin.
So, what should catholics who want to space out their babies do?
If therefore there are well-grounded reasons for spacing births, arising from the physical or psychological condition of husband or wife, or from external circumstances, the Church teaches that married people may then take advantage of the natural cycles immanent in the reproductive system and engage in marital intercourse only during those times that are infertile, thus controlling birth in a way which does not in the least offend the moral principles which We have just explained. In plain English, married people can have sex during infertile days and still avoid pregnancy.
But if you let a piece of rubber contain your ejaculation, you are engaging in morally evil sex. If you consume a birth control pill which stops your ovary from releasing a mature ovum, then that would be morally wrong.
But, if you knowingly have sex only on your infertile days, you are somehow still "open" to procreation and you have the Church's blessings!
Pope Paul VI should have taken in a high school level course in logic, perhaps!
Invitro fertilization (IVF)For some parents who have trouble conceiving, invitro fertilization is an option. The ovum is removed from a woman's body, and the fertilization happens on a petridish/test tube. The fertilized embryo is then reintroduced into the woman's body where it continues its nine month journey.
The problems are multi-faceted. In IVF, the semen is often produced by masturbation which is of course evil in the eyes of god. Then the fertilization happens outside the body of the woman which is just not natural and hence is evil. Finally, the 'marriage act' is being replaced by an artificial process. Procedures which add a "third party" into the act of conception, or which substitute a laboratory procedure for intercourse, are not acceptable, the USCCB clarifies for us.   
The Congregation for the Doctrine of the Faith (CDF) in its document 'Donum Vitae' issued in 1987 teaches that if a given medical intervention helps or assists the 'marriage act' to achieve pregnancy, it may be considered moral; if the intervention replaces the 'marriage act' in order to engender life, it is not moral.
The USCCB has this to say about IVF:
... the procedure does violence to human dignity and to the marriage act and should be avoided. In place of IVF, GIFT or LTOT are possibilities that are not as morally wrong. Gamete Intrafallopian Transfer (GIFT) is actually under consideration by the Sacred Congregation for the Doctrine of the Faith (CDF). Until the CDF makes up its mind, it is left to you. Low Tubal Ovum Transfer (LTOT) a.k.a Normal Egg Sonographic Transfer NEST (NEST) is the other option.
They both involve the couple having sexual intercourse. So, no masturbation is involved. Whew! Then the egg and the semen are retrieved, and reintroduced back into the woman's body so that fertilization happens inside the body and not outside in a test tube, which would of course be evil.
In cases like IVF or GIFT, multiple pregnancies are very common. Pregnancy reduction or aborting some of the fetuses is absolutely not an option even though it gives the remaining embryos a better chance of survival. Any eggs that have not been used should absolutely be not fertilized invitro.
Well, in short, this is an elaborate tap dance around what IVF does. And why the church thinks this procedure does not involve a "third party" into the act of conception is any body's guess. They must also be working on a definition of 'natural' that the rest of us are unaware of.
They make up 'divinely inspired' rules and then find loop holes and skirt around the rules.
Here is what I think is the real moral evil here - trying to play god and throwing a monkey wrench into the lives of over a billion people.
We can blame the CDF or the Pope for all of this. But Catholics who silently stand by, use contraceptives and let the church get away with all of these terrible and not to mention, stupid rules must take a significant portion of the blame as well. Without their silent help, the church would not get away with all of this nonsense.
More Divine Loopholes: Abortion.
Footnotes Natural Family Planning
 HUMANAE VITAE: ENCYCLICAL LETTER (1968)
 INSTRUCTION ON RESPECT FOR HUMAN LIFE IN ITS ORIGIN AND ON THE
DIGNITY OF PROCREATION REPLIES TO CERTAIN QUESTIONS OF THE DAY (also
known as 'Donum Vitae') 1987.
 INSTRUCTION DIGNITAS PERSONAE ON CERTAIN BIOETHICAL QUESTIONS (2008)
 Begotten Not Made: A Catholic View of Reproductive Technology.
 Contraceptive Methods Women Have Ever Used: United States, 1982–2010