A more accurate statement would be: If science hasn't yet found conclusive genetic evidence of homosexuality, wouldn't it be irrational to deny the possibility?
To date, studies are inconclusive, yet there are strong indicators for a combination of genetic and environmental causes for same sex attraction, not only in H. sapiens but also in many other species.
Same-Sex Behavior Found in Nearly All Animals LiveScience June 2009
Examples of same-sex behavior can be found in almost all species in the animal kingdom — from worms to frogs to birds — making the practice nearly universal among animals, according to a new review of research on the topic.
Plenty O' Partners LiveScience Sept 2011
Homosexuality has been documented in more than 450 species of vertebrates signaling that sexual preference is biologically determined in animals. From Male bonobos that hang from trees and engage in "penis fencing" to bull manatees and bottlenose dolphins, the animal kingdom tolerates all kinds of lifestyles.Do Gay Animals Change Evolution? Scientific American June 2009
Animals that engage in same-sex sexual behavior may be acting in accord with adaptational strategies rather than against them--and bending the way we think about evolution