"God created man in His own image... Male and female He created them" (Gen. 1:27). He blessed them and said "Be fruitful and multiply" (Gen. 1:28). This means that any sexual acts that are not open to life, are against God & therefore a sin. Since homosexual acts can never be fruitful (because there is no complementarity of the sexes) it goes against God's commandment. This teaching also applies to contraception and masturbation.