What is the basis for Catholic teaching about the immorality of homogenital acts?
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 contraception, masturbation, and pre-marital and extra-marital sex as well as homogenital acts.
The Vatican insists that procreation 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.