פירוש על עבודה זרה 46:8

The red heifer is rendered unfit if someone has sex with it not because it is considered to have the holiness of a sacrifice offered on the altar, but because a physical flaw disqualifies it. Through a series of connections drawn from the word “corruption,” the Talmud derives that just as a flaw disqualifies the red heifer, so too does bestiality.