I do not have any references about this, but many Western laws have been carried over from our Jewish/Christian backgrounds. Bestiality is forbidden in the Bible numerous times, e.g. in Exodus 22:19:

Whoever lies with an animal shall surely be put to death.

As for the need for these laws: one thing that comes to mind is that intercourse with an animal without proper protection can result in new sexually transmitted diseases, causing not only danger to the offender's health but also to other people.
Under some (all?) conditions it's animal abuse, but there are separate laws for that.

I do not have any references about this, but many Western laws have been carried over from our Jewish/Christian backgrounds. Bestiality is forbidden in the Bible numerous times, e.g. in Exodus 22:19:

Whoever lies with an animal shall surely be put to death.

As for the need for these laws: one thing that comes to mind is that intercourse with an animal without proper protection can result in new sexually transmitted diseases, causing not only danger to the offender's health but also to other people.
Under some (all?*) conditions it's animal abuse, but there are separate laws for that.

*_{: I've been told that e.g. cows don't feel anything from the act; after all, sometimes a vet inserts their entire arm into a cow's rectum during an examination. I don't think it's necessary to discuss this; I usually try to avoid the subject.}