You might think it's a crazy law professor hypothetical, but it's a real case. Michigan Compiled Laws § 750.158 provides,

Any person who shall commit the abominable and detestable crime against nature either with mankind or with any animal shall be guilty of a felony ....

Under Michigan law, the "abominable and detestable crime against nature" meant anal sex when committed with humans (which can now no longer be made criminal, with the usual provisos related to consent, age, lack of relationship, noncommerciality, and privacy), and any sort of sex when committed with other animals. Michigan law also provided that people register as sex offenders when they have been found guilty of "[a] violation of ... MCL 750.158, if a victim is an individual less than 18 years of age." Hence the question for the reader:

Under Michigan law, someone who has sex with a sheep must register as a sex offender:

1. Always.
2. Never (unless he's guilty of some other crime for which he must register).
3. Only if the sex is anal sex.
4. Only if the sheep is under eighteen.
5. Only in a rural county.

For the answer, read today's People v. Haynes. Thanks to How Appealing for the pointer.