To answer why homosexuality exists we must first identify it. "Homosexuality" has two principle meanings which are distinct but related. Homosexuality is sexual activity between two individuals of the same gender. This includes same-sex coitus or genital contact, whether ventro-ventral, dorso-ventral, or otherwise and whether male-male or female-female.
Homosexuality is also the innate sexual preference for one's own gender or the biological urge for same-sex coitus. While much is said about the distinction between homosexual desires (or innate sex drive) and homosexual behavior in humans, this distinction is of limited or negligible significance in non-human populations in which we infer "desire" (be it instinctive or learned) primarily from behavior. The distinctive gulf between human desire/biological impulse and behavior (Marmor, 1980) is attributed to human social mores and cultural constraints (Dickemann, 1993), which epigenetic forces can also be accounted for with biological/evolutionary explanations (Alcock, 1984, pp. 522 & 524).
In summary, homosexuality is both the biological drive for same-sex coitus as well as the performance of same-sex coitus. Homosexuality (as defined by both definitions) has been observed in both males and females in both human and non-human populations. While it is difficult, if not impossible, to separate innate urge from action in non-human animals, the distinction between these two definitions will be important later as we discuss possible evolutionary explanations for homosexuality.