Human beings are living creatures, therefore they copulate. The same goes with animals, as all we know, but we're not completely sure whether they do it for the same reasons. For a long time, science kept saying that procreation is the only reason why animals have sex, as if humans were the only living species on earth that have sex for enjoyment too. However, scientists are now collecting evidence that animals, and especially mammals, experience sex in ways that relate also to more immediately relevant mental and emotional experiences. An ape have sex because it's driven by a natural instinct, it will never wonder if it's appropriate or not to copulate for not procreation-related reasons. Animals just have sex, end of the story, they can have social habits but not moral constraints.

The ‘hippie apes', or Bonobos, are the most famous example: sex is a constant in Bonobo's social life and it's fundamental for these apes to form social bonds. Bonobos have no sex taboos on gender, age or social class and, quoting Wikipedia, "Bonobos are the only non-human animal to have been observed engaging in all of the following sexual activities: face-to-face genital sex (though a pair of western gorillas has been photographed performing face-to-face genital sex too), tongue kissing, and oral sex.". It is also quite interesting to observe that Bonobos are genetically as close to humans as chimpanzees, but their society is matriarchal and, in general, they aren't aggressive animals, while chimpanzees can often be very violent with each other.
The American science writer Jason G. Goldman recently wrote a piece on bbc.com about sex behaviours in animals, highlighting some interesting recent discoveries. For instance, researchers Alfonso Troisi and Monica Carosi spent 238 hours watching Japanese macaques - observing around 240 individual copulations (!) - and came to the conclusion that females can have a form of orgasms and that usually happens when copulating with males belonging to an 'higher social class'. Isn't it curious? Check also the following links, where you can find some more interesting stuff about our beloved Bonobos and gay-ish brown bears.
What we are saying now could get many scientists angry because one of the most important things to keep in mind when you talk about animal observation is to not follow the temptation of saying "Oh god! They are animals but they are acting like humans": ‘anthropomorphosis' isn't a scientific attitude. Nonetheless, the article by Goldman ends with quoting ethologist Jonathan Balcombe, who, in the Applied Animal Behaviour Science, seemed to conjugate a canonical Darwinian point of view with some interesting consideration about animals and sex. "Pain's unpleasantness helps steer the animal away from 'bad' behaviours that risk the greater evolutionary disaster of death. Similarly, pleasure encourages animals to behave in 'good' ways, such as feeding, mating, and...staying warm or cool."