If it is heritable, then it would have to negatively affect reproduction rates to be incompatible. As you, of course, know, evolution is not about individuals but populations. Homosexuals may contribute to the upbringing of their sibling's children, thus positively affectiving reproduction rates in the population. In any case, many predominantly homosexual animals, including humans, still reproduce.
Additionally, even if one wished to characterise homosexual behaviour as evolutionarily maladaptive, there are many examples of maladaptive traits that have evolved on the back of useful ones; for example the same gene which gives many resistance to cholera and thus is selected for, is also responsible for cystic fibrosis.
Further, if homosexuality is not a heritable trait but a socially determined one, then it may simply be too recent to have been selected for by evolution, (particularly if you consider the fact that in humans we regularly choose to bypass evolutionary pressures through medicine and technology - we are no longer entirely subject to it).
Many animals exihibit homosexual behaviour for reinforcing social bonds, and far from possessing no societal benefits are significant factors in social cohesion.
In short, the question has already been extensively considered, and has many possible answers, however, until we know what causes homosexuality for sure, we cannot say for definite how it fits in with evolution. Whichever way it turns out, there's nothing to suggest any incompatibility with evolution.