Sefer HaChinuch ספר החינוך

To not lay with a beast: To not lay with a beast — whether the man had intercourse with the beast or brought the beast upon himself, it is all included (Sanhedrin 54b); and also included in beast is a [wild] animal — as it is stated (Leviticus 18:23), “And to any beast you shall not give your laying.”

It is from the roots of the commandment that God, blessed be He, desired that all the species that He created in His world make offspring according to their species and like it appears in Scripture at the beginning of the Creation, such that it is written with each and every species, “according to its species.” And since it is His will like this, blessing and greatness will not be found in that which is the opposite — as is known to us with mules that come from a mixture of two species, that they do not become fruitful and multiply. And so [too,] any tree that is grafted from two species will not be successful to make fruit like it. And all the more so with man — who is select from all of the species — is it not fitting to mix with a species of low and inferior beasts.

From the laws of the commandment is that which they, may their memory be blessed, explained (Sanhedrin 54b) that there is no distinction between a large beast and a small [one], as it is stated, “And to any beast” — meaning to say, even on the day of its birth. And there is no difference whether he has intercourse with it in its way, or not in its way. In any fashion, once he has inserted himself in it — meaning to say, when he inserted the tip of the corona — or the beast inserted itself into him, they are both stoned, he and it. [This is] if he is an adult — meaning to say he has reached the category of punishments — and he was volitional. But if inadvertent, the animal is not stoned. And if he is from nine years old and a day and up, the animal is stoned, but he is exempt until he reaches the category of punishments, but it is nonetheless fitting to chastise him. However if he is less than nine years and a day, even the animal is not stoned because of him. And we will write the place of all the rest of its details at the end of the Order, with God’s help.

And this prohibition is forbidden in every place and at all times. And one who transgresses it volitionally and there are witnesses and a warning is stoned, as we have said; but if there are no witnesses, it is [punishable] with excision. And if inadvertent, he is obligated to bring a fixed sin-offering at the time of the [Temple].