Many species other than humans have sex for pleasure. Although we can never know precisely what degree of pleasure is experienced, it is clear that sex serves for far more than just procreation. One of its greatest benefits may be called social lubrication, as it reduces friction, and bonds individuals. Bonobos are best known for this.
Bonobos are the bimbos of the animal world, but only if one considers the label has no negative connotation. They make love to avoid aggression, for developing social relationships, play, and pleasure. Would that human primates could more often make love and not war.
Here are some non human animals that are best known to enjoy sex in all of its many manifestations: Dolphins, rats, primates, cats, horses, and almost all avian (bird) species.
There are other clues that suggest sex for pleasure is the norm in biology. Many species have methods of masturbation. Birds prefer stalks of grass, or mounds, and other animals use a variety of tools, including their own auto-erotic bodies. Orcas, deer, and penguins are among animals most studied in masturbation behavior.
Many animals practice bisexuality, homosexuality, and varying degrees of polygamy and monogamy, not just for its evolutionary advantage, but also for recreation. All mammals that are female are born with a clitoris, (and only a few examples of one dim animal is known to insanely remove these), and a clitoris, the specialized, female version of a penis, appears to be designed purely for increasing ease of mating, and pleasure.
Animals will make sexual displays, sometimes to indicate dominance, but there is no reason to believe they do not also experience pleasure, and practice forms of play in order to learn mastery of such displays. Even courtship, elaborate and refined, such as done by the artistic Bower birds, could be considered very pleasurable, even anticipatory of sex.
When a dog “humps” a human leg, there is no reason to believe the dog is doing it to make puppies. There are thousands of examples human beings are less exposed to, of course, as we most often live our lives quite apart from the other animals.
Sex between species, which includes many levels of hybridizations, often produces offspring that are infertile. A Mule is the best known example, but recently, we have Ligers, and Zeedonks, and Dzo, oh my! Although many examples are recent results of tinkering by humans, it is Nature who of course first invented the idea of cross breeding.
A Liger is the offspring of a lion and tigress, a Zeedonk the offspring of a donkey and zebra, and the Dzo is the male offspring of Yak and cattle.
Other forms of sexual behavior less common, but still observed, in animals include response to pornography, (although visual imagery seems a less loaded term) sexual fetishes, and juvenile sexual behaviors with peers.
One of the more curious aspects of sex for pleasure is that a male penguin will offer pebbles, the currency of penguin culture, (more tangible than our silly electronic banking systems), and female penguins will “prostitute” themselves to win these nesting gems. This does not affect the monogamy of the primary mated pair, as all involved apparently gain something.
In the animal kingdom, nature always works to increase diversity, increase robust gene pools, and devise many ways for behaviors to benefit organisms.
When people speak of animals mating only for procreation, they are expressing views, not facts, often with the underlying idea that sex for pleasure is somehow perverse.
Sex for pleasure is natural, and occurs to that life itself occurs.