It’s a very difficult question. We can’t get inside the brain of an animal and see whether they find mating pleasurable – or even whether they experience “pleasure” in the same way that we do. So what do we know? Well, we know that humans frequently mate for pleasure. And we know that humans evolved just like everything else in the living world. From an evolutionary perspective it would make sense that animals feel some pleasure in mating. Think about it like this: imagine two animals, one of which has a gene that makes it pleasurable to mate, and the other which doesn’t. The one that gets pleasure from mating is more likely to mate, and so is more likely to have offspring. So the genes that cause pleasure when mating will get passed on. The one that doesn’t get pleasure is less likely to mate, and so is less likely to have offspring. So the genes that don’t cause pleasure in mating will be less likely to be passed on. Therefore the genes that cause it to be pleasurable to mate are more likely to stick around, and so it is likely that animals do get some pleasure from mating. This isn’t to say that it can’t be instinct as well as pleasure.
But then we also know that humans have much more complicated brains that everything else. It doesn’t seem sensible to say that, for example, plants feel pleasure. But they do mate. My guess would be that animals with brains that are developed enough to feel pleasure probably do feel some sort of pleasure in mating. But that’s mainly my opinion. It’s hard to get full scientific facts about what animals do or don’t feel because we can’t ask them. Most research into what animals “like” or “don’t like” just looks at what activities they do when they’re given the opportunity, and what activities they avoid. In most species, mating is certainly something animals do when given the opportunity. So I think it’s probably instinct and pleasure.