I can think of a lot worse things that would warrant putting someone in prison. Most of the laws against zoosexuality are impossible to enforce as is (because how many people f**king horses are doing so in broad daylight?) and putting another person engaged in sexual activity with an animal in jail is ultimately committing ourselves to maintaining another person in an overcrowded prison system that more or less turns minor criminals into major criminals. I don't think it's worth the time or effort to prosecute people for engaging in these kinds of acts, largely because the population involved is insignificant and they're not really hurting anyone.
It's still f**ked up though.
I suggest we solve this problem by summarily executing people who animals. I don't need some kind of high-minded philosophical justification for this because ew wtf.