Reproduction is important for a species, but thats not the only benefit of sex. For example see Bonobos:

https://en.wikipedia.org/wiki/Homosexual_behavior_in_animals

Homosexual behavior in animals is sexual behavior among non-human species that is interpreted as homosexual or bisexual. This may include sexual activity, courtship, affection, pair bonding, and parenting among same-sex animal pairs. Research indicates that various forms of this are found throughout the animal kingdom. As of 1999, about 500 species, ranging from primates to gut worms, have been documented engaging in same-sex behaviors. According to the organizers of the 2006 Against Nature? exhibit, it has been observed in 1,500 species.

Bonobos, which have a matriarchal society, unusual among apes, are a fully bisexual species—both males and females engage in heterosexual and homosexual behavior, being noted for female-female homosexuality in particular.

Roughly 60% of all bonobo sexual activity occurs between two or more females. While the homosexual bonding system in bonobos represents the highest frequency of homosexuality known in any species, homosexuality has been reported for all great apes (a group which includes humans), as well as a number of other primate species.

Dutch primatologist Frans de Waal on observing and filming bonobos noted that there were two reasons to believe sexual activity is the bonobo's answer to avoiding conflict. Anything that arouses the interest of more than one bonobo at a time, not just food, tends to result in sexual contact. If two bonobos approach a cardboard box thrown into their enclosure, they will briefly mount each other before playing with the box.

Such situations lead to squabbles in most other species. But bonobos are quite tolerant, perhaps because they use sex to divert attention and to defuse tension.

Bonobo sex often occurs in aggressive contexts totally unrelated to food. A jealous male might chase another away from a female, after which the two males reunite and engage in scrotal rubbing.

Or after a female hits a juvenile, the latter's mother may lunge at the aggressor, an action that is immediately followed by genital rubbing between the two adults.

And swans:

An estimated one-quarter of all black swans pairings are of homosexual males. They steal nests, or form temporary threesomes with females to obtain eggs, driving away the female after she lays the eggs.

More of their cygnets survive to adulthood than those of different-sex pairs, possibly due to their superior ability to defend large portions of land. The same reasoning has been applied to male flamingo pairs raising chicks.

Basically having 20% or so homosexual leads to lower infant mortality which is a net gain for the species so its selected for, with other species like Bonobos and primitive hunter / gatherer humans sexuality is a good way to strengthen bonds and reduce tensions in the absence of the ability to do so with language and other communication