How can homosexuality evolve despite natural selection?
Several years back, results of a twin study (Zietsch, et al., 2008; popular press in The Economist) suggested that genes associated with homosexuality make heterosexuals who carry the same genes more reproductively successful. This would explain the observation that, for example, sisters of homosexual males have more offspring.


  We show that psychologically masculine females and feminine men are
  (a) more likely to be nonheterosexual but (b), when heterosexual, have
  more opposite-sex sexual partners.


So if more sexual partners in heterosexuals is correlated with more offspring, then the genes associated with homosexuality may lead to more offspring in heterosexuals that carry them. So these "homosexuality" genes could easily be maintained in the population by conferring a reproductive advantage to siblings.

The authors conclude:


  Taken together, these results suggest that genes predisposing to
  homosexuality may confer a mating advantage in heterosexuals, which
  could help explain the evolution and maintenance of homosexuality in
  the population.