Do tortoises fuck?
There is a principle in Jewish law called "marit ayin" which says that an action which is permissible in theory but which looks sufficiently like an impermissible action that they are not distinguishable is also impermissible. For instance, animal blood is not kosher and cannot be eaten. Fish blood, on the other hand, is kosher. But under the principle of marit ayin, fish blood cannot be eaten, because someone eating fish blood looks exactly like someone eating animal blood.
While tortoises could in principle fuck, people have trouble telling the difference between turtles and tortoises, and might, seeing tortoises fucking, assume they were turtles and wrongly conclude that turtles fuck. Therefore, it is clear that, by marit ayin, tortoises, while they could fuck, are forbidden to do so.