First issue: Abortion
#2270 "Human life must be respected and protected absolutely from the moment of conception. From the first moment of his existence, a human being must be recognized as having the rights of a person- among which is the inviolable right of every innocent being to life.
Jere. 1:5 "Before I formed you in the womb I knew you, and before you were born I consecrated you."
Ps.139:15 "My frame was not hidden from you, when I was being made in secret, intricately wrought in the depths of the earth."
Second issue: birth control/contraception
#1652 "By its very nature the institution of marriage and married love is ordered to the procreation and education of the offspring and it is in them that it finds its crowning glory."
-Children are the supreme gift of marriage and contribute greatly to the good of the parents themselves. God himself said: "It is not good that man should be alone," and "from the beginning [he] made them male and female"; wishing to associate them in a special way in his own creative work, God blessed man and woman with the words: "Be fruitful and multiply." Hence, true married love and the whole structure of family life which results from it, without diminishment of the other ends of marriage, are directed to disposing spouses to cooperate valiantly with the love of the Creator and Savior, who through them will increase and enrich his family from day to day." (Gen.2:18, Mt.19:4, Gen.1:28)
Third issue: embryonic stem cell research
#2274 "Since it must be treated from conception as a person, the embryo must be defended in its integrity, cared for, and healed, as far as possible, like any other human being."
Final issue: gay marriage
#2357 "Homosexuality refers to relations between men or between women who experience an exclusive or predominant sexual attraction toward persons of the same sex. It has taken a great variety of forms through the centuries and in different cultures. Its psychological genesis remains largely unexplained. Basing itself on Sacred Scripture, which presents homosexual acts as acts of grave depravity (Romans 1:24-27, 1 Cor.6:10, 1 Tim.1:10), tradition has always declared that "homosexual acts are intrinsically disordered." They are contrary to the natural law. They close the sexual act to the gift of life. They do not proceed from a genuine affective and sexual complementarity. Under no circumstances can they be approved."
#2359 "Homosexual persons are called to chastity. By the virtues of self-mastery that teach them inner freedom, at times by the support of disinterested friendship, by prayer and sacramental grace, they can and should gradually and resolutely approach Christian perfection."
When you call yourself Catholic the world sees you and assumes (sometimes wrongly) that you are practicing what the Catholic Church teaches. You can and do set a bad example and can become a stumbling block for others when you live your life in opposition to the faith you claim. Now for the big divider during election years- Do Catholics have to vote according to their faith? Well, if you don't you're a hypocrite aren't you? If you don't believe what the Church teaches, why are you calling yourself a Catholic?