To the extent that philosophers have felt the need to argue against sex with animals at all, the most common strategy has been to appeal to the fact that these beings lack life projects, and thus that a sexual relation with them cannot amount to a shared life project with a human being. It is further presumed, without argument, that any morally praiseworthy sexual relation ought to be such a project.
Something like this account is often heard in response to the conservative complaint that to accept homosexuality in our society will lead quickly to an 'anything goes' atmosphere in which bestiality, among other perversions, thrives. As former congressman Rick Santorum worried, or pretended to worry, in 2003, once you've got man-on-man sex, why not man-on-dog?
In a 2005 article (“Homosexuality and the PIB Argument,” Ethics 115 (April, 2005): 501-534), the philosopher John Corvino responds to Santorum's reasoning with a lengthy account of the various respects in which same-sex activity differs from what has come to be called 'PIB', that trifecta of unacceptable sexual relations: pedophilia, incest, and bestiality. Corvino would keep bestiality in its traditional place, while promoting same-sex relationships from their traditionally marginal place into a mainstream one. His argument is based largely on the claim that sexual contact with an animal cannot, by definition, contribute to a profound interpersonal interaction, while a same-sex, intraspecies relationship is as well-suited to do so as a heterosexual one.