There are traits, which are unfavorable to the individual in terms of spreading his genes, however, benefitting the population. Homosexuality can only be assumed to be unfavorable in terms of spreading one's own genes and also only if it is exclusive (cf. Ancient Greece).
Since homosexuality is not proven to be solely based on genetics, it is not entirely true to call it a trait. A biological trait is purely genetic and/or epigenetic. Thus the premise is not worded correctly.
"Advantageous traits (that help certain individuals survive and reproduce) are said to be 'selected'"(Assuming selected=favourable) Homosexuality has long been punished by environmental factors and thus not been favourable to survival.So at least one condition to call homosexuality favourable fails
From a historical societal standpoint, it is also undesirable. Which begs the question: From what group standpoint is homosexuality desirable? I request a group standpoint because from individual standpoints, any number of arguments can be made regarding any topic.
maybe we should ask how strong we understand homosexuality: - only preference for mens (include sex for children with women) - only have sex with men (exclude sex for children with woman) ...however this is only relevant for the non-humans and non-aliens. Today we can adopt easily...