There’s an idea circulating that humans are the only animal to experience sexual pleasure; that we approach sex in a way that is distinct from others. As with many questions about sex, this exposes some interesting facts about the way we discuss the subject.
On one level, the question of whether humans and nonhumans experience sex in the same way is fairly simply dismissed: how would we know? We cannot know how a nonhuman experiences anything — they can’t be asked. Sex as an experiential phenomenon for nonhumans is, quite simply, inaccessible. Science is obliged to propose questions that are answerable, and “how does a leopard slug experience sex?” is, at time of writing, about as unanswerable as they get.
Having said that, we can make educated guesses about whether sex is pleasurable for other species. Sex would be a very strange thing to seek if it didn’t bring some form of pleasure. It increases risk of disease, it wastes energy, it can seriously increase the likelihood of something bigger coming along and eating you (seriously, check out leopard-slug reproduction, below).