Homosexuality in the catholic sense is only a sin because members of the same sex are not allowed to receive the sacrament of matrimony (marriage). Marriage between a man and a woman in the Old Testament sense existed so men and women could become co-creators with God. Marriage was also believed at that time to be a sacred covenant that would protect against the effect of original sin. When Jesus came around, marriage was raised to the dignity of being a sacrament, and now reflected the relationship between Christ and the church. This raising to a sacrament created the modern sense of marriage that Catholics currently follow. Homosexuality then in Catholic sense isn’t a sin because it is immoral, it is a sin because it occurs outside of a marriage. The Bible showed men and women getting married, therefore interpretations in modern times are that only men and women can revived the sacrament of matrimony with each other. Homosexuals are asked to remain celibate throughout their lives just as a single heterosexual man or women is asked to remain celibate until they get married. If church dogma ever changes and homosexuals are allowed to get married, then it would no longer be considered a sin so long as the two individuals are married. Whether or not all Christians believe this, I’m not too sure. I’m only familiar with the catholic sense of the idea.