I found this buried in my saved threads. There's certainly more to be said, but you might be interested in an answer I provided to a similar question.

I'll also add a little bit here to try and answer "why" because I'm willing to guess up front that the provided explanation of "because it makes men's soles into demons and false gods" is probably unsatisfying.

Within Zoroastrianism, and especially the context of the Vendidad I discuss in that link, the dichotomy of life and death is absolutely essential. Life is where good things are able to occur and death is a product of pure evil. Dead matter, and in fact almost any matter that leaves the body, was considered deeply unclean. That covered everything from blood to excrement, to toenail clippings. Semen on the other hand necessarily outside of that category because of its role in procreation. The answer linked above discusses prohibitions against both male-male sex and non-procreative ejaculation more generally, and specifically addresses punishment for unintentional ejaculation as well, meaning its not just moral objection to the "lustful" context of masturbation. In a society that valued creation of life and condemned basically anything that left the body, male homosexuality violated both of the basic moral rules.