All Catholic sexual ethics rests on this principle: procreation is an essential aspect of human sexuality, so every genital act must be open to the possibility of conception. For this very same reason Catholic teaching forbids homogenital acts as well as contraception, masturbation, and pre-marital and extra-marital sex.
This teaching pertains to the very nature of human sexuality. That is, the Church presents this teaching as natural law, the ordering which the Creator built into the universe.
Taken from Daniel A. Helminiak, FAQs: Catholicism, Homosexuality, and Dignity, written for DignityUSA, 1996, 2000.