Even cross generational sex is engaged in with infant males copulating with mature females and mature males having sex with adolescent females. Females often take all comers, whether they are experiencing estrus or not. Sex is definitely NOT done solely for procreation. While monogamy exists in some species, it is not a universal. And while procreation might be the primary reason for sexual behavior it is not the only one.
Sex is engaged in for pleasure and socialization as well.
It's unrealistic to draw behavioral parallels from the animal kingdom. And in fact, it is NOT true that homosexual behavior is absent among animals. Whatever two or more adults decide to do, consentually, is fair game. How They Do It, by Robert A. Wallace, William Morrow and Company, Inc., New York, 1980. References are collected in Weinrich, pp.