Although there may be social reasons for ignoring and/or condemning zoosexuality, one reason for the taboo of zoophilia may be evolutionary; from a biological point of view, having sex with animals makes no sense because it doesn’t result in any offspring; consequently, the only humans that survived were humans that had sex with other humans, and because of this it became hard-wired into the DNA of their descendants (ultimately, most humans). This might explain why when someone is approached with the idea of zoosexuality, their initial reaction might be “eww, that’s gross” or “that’s perverted”. It’s because they’re not used to such “abnormal” ideas because society has told them it’s wrong and they are biologically programmed to do so. However, keep in mind that saying “that’s gross” is not a philosophical argument; it does not rely on any rational reasoning system to determine whether or not zoosexuality is moral. There are rational arguments that could be made for or against zoosexuality; for example, a utilitarian argument for zoosexuality is that it increases happiness so long as both parties are satisfied.