Why do some people have sex with dogs?

Some persons who have sex with dogs do so because they are “contact-zoophiles” who are sexually attracted to animals. However, actually following through on this sexual attraction and perpetrating bestiality is only ever due to them not seeking treatment for their paraphilia and psychiatric disorder. According to the DSM-5, It is a paraphilia and psychiatric disorder because, while their sexual attractions to animals may not be distressing to them, any sexual thought that is acted upon and results in bestiality also results in behavior that causes harm to sentient beings who cannot consent. Unlike what was stated in another response, all contact-zoophiles are zoosadists because bestiality is animal sexual abuse and is, both, emotionally and physically harmful to any animal that is sexually abused.

Having sex with dogs/animals goes well beyond just being “weird”, as stated elsewhere. Lots of people are weird/eccentric/quirky, but that does not at all mean that they will sexually abuse an animal. Sexually abusing an animal is due to an underlying psychiatric or personality disorder and anyone who engages in bestiality requires both psychological and medical intervention, i.e. therapy and medication. However, zoophilia and bestiality are under-studied/under-researched areas and treatment for bestiality sex offenders is not an exact science. Most bestiality sex offenders are treated similarly to pedophiles and other sex offenders with paraphilias and/or psychiatric disorders. Some who are motivated to change and seek out treatment themselves may be rehabilitated without the use of medication.

There are also some persons who have personality disorders, such as sociopaths, who do not feel empathy or remorse and are more likely to be cruel to animals. Most zoosadists who sexually abuse animals, without being sexually attracted to them as their perceived “sexual orientation”, will fall under this category. They find pleasure in causing an animal pain. They have sex with dogs/animals because they are sexually violent predators and also have untreated disorders.

Secret leprosy infected the Americas before European arrival - DW