Hello! I'm Catholic, was raised by a Catholic family, and attended Catholic school. So I think I can elaborate on this.
For the record, I do not agree with everything my Church teaches. (I'll probably get bashed for saying that, but I don't care. ) I remain Catholic because I admire the principles it-- and the rest of Christianity-- were founded upon: good will towards others, charity, selflessness, servitude, non-violence, etc. Jesus taught wonderful things, created a wonderful Church, and I believe that humans have messed it up over the centuries.
Going on, there are many different justifications Catholics teach for being against homosexual
acts. I bolded "acts" because the distinction is important-- Catholicism does
NOT teach that having romantic feelings for someone of the same gender is bad or sinful(though many interpret it that way). What Catholicism does condemn is sexual intercourse between people of the same gender. Because many couples include sex in their relationships, Catholics tend to be against homosexual coupling and especially same-sex marriage.
Why do Catholics condemn sex between two people of the same gender? Several reasons.
1. The Bible has many passages that tell of people who were punished by God for committing acts of sodomy. People do not want to anger God, so they avoid and try to eradicate anything they think would be against His laws.
2. Humans were created so that men and women were two pieces of one whole. One completes the other. A spiritual marriage (the Sacrament of Matrimony) is a promise and a symbol of that bonding, as is the consummation of that relationship-- aka, sex. This is impossible for people of the same gender, so a "true" marriage is impossible. It's not that Catholics bar marriage to homosexual couples out of hate; it is that homosexual couples do not meet the requirements for a spiritual marriage. I know many Catholics who are fine with homosexual couples having legal marriages. That doesn't matter at all. However, there are many others who equate legal marriage to spiritual marriage, and therefore we have problems.
3. Sexual intercourse, as defined in the Bible, is not just an act of love or lust. Its main purpose is procreation. It is supposed to have a child as its result. I know that's not how people see it, both today and in thousands of years past, but that is the moral taught to Catholics. Speaking simply, sex between a man and women can result in a child. Sex between those of the same gender cannot. One of the goals of marriage to have a child. Thus, since two people of the same gender cannot have a child (naturally), they cannot get married. If you have sex and you are not married, then it is considered to be adultery. Adultery is forbidden. Thus, if homosexual acts are acts of adultery, then homosexual acts are forbidden.
4. Other reasons I have forgotten.
I hope that sheds some light on Catholic beliefs. There are many fine details that go into the above reasons, but I gave the simple version. If another person has more information and wants to elaborate or correct me, feel free to do so.
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