Let me mention something else that non-Catholics are usually not aware of regarding Catholic teachings:
There are five levels of Catholic teaching.
At the top is what we call the Deposit; this consists of the original teachings that Christ gave the Apostles. It was preserved in two forms: Holy Scripture and Apostolic Tradition. Apostolic Tradition was passed from bishop to bishop orally for about two hundred years, and eventually was all recorded in the writings of the Patristic Fathers, the early Christian leaders during that period. The Deposit is Divinely insipired, it is infallible, and it cannot be altered or changed.
Next down on the list is Dogma. Dogma is a definition of, or a clarification of, some doctrine or controversial point. It comes from two different sources: the assembled bishops in a ecumencial council issue it in a pronouncement; or a reigning Pope makes an infallible statement. A Dogma is also infallible, and it also cannot be changed; it also cannot contradict the Deposit or a previously defined Dogma. It should be noted that when a Pope makes an infallible statement, he must be speaking about a matter of faith and morals, he must be speaking in the role of universal bishop of the Catholic Church, and again, what he says cannot contradict the Deposit, previous Dogma, or a previous infallible statement. An example of a Dogma would be the definition of the Holy Trinity.
Next comes Doctrine. This is an explanation of a teaching. It is not infallible, and it may be changed, abandoned, condemned, resurrected, what have you. An example of a Doctrine would be the concept of limbo, which was abandoned after the 2nd Vatican Council.
After that comes Discipline, which is basically a practice the Church imposes for one reason or another. It is not infallible, and can be changed if the Church sees fit. Examples of this would be clerical celibacy and fast days during Lent.
Last comes Devotion, which is a system of prayer or practices that may be employed by the individual believer. It is also not infallible and may be used or abdandoned at will. Examples of this would be the Rosary and virtually all Marian apparitions/devotions.
So, while lower levels of Catholic teaching have changed from time to time throughout history, nothing in the top two rungs have changed, nor will they. In the cases of abortion, contraception, female clergy, and active homosexuality, for example, they all fall under the bans found in Apostolic Tradition, which, as was previously mentioned, is infallible and cannot be changed.