Hensley, Adams, and Zillman extend the theory that bestiality like other forms of animal cruelty is predictive of psychopathy and violence against humans.
Peter Singer is the most prominent advocate for only making it a crime when there is actual cruelty to the animal on a case by case basis, with criticism of him in papers commenting on his work. Critics on the animal cruelty front such as Beirne are largely arguing that animals can't meaningfully consent.
The linksRubin, Monter and Rydström, also indicate the linkage of LGBTQ issues to Bestiality, in general, in legal/political motive.
Several look at historical status like Murrin, Salisbury, Monter, Rydström and Liliequist are applicable to the reasons for older wave statutes, basically moral ones motivated by "disgust" and traditional morality.