Is homosexuality an adaptation in humans?
Barnard's wonderful text "Animal Behavior: Mechanism, Development, Function and Evolution" offers some enlightening summary to the discussion of homosexuality in a biological context!

Brief Summary to ch 12, page 606:


  What we know so far:
  
  
  homosexual behavior is recorded all throughout animal kingdom
  there are evidence for genetic basis to homosexuality in various species including humans
  
  
  Three interrelated schools of thoughts:
  
  
  Homosexuality is maladaptive - external stress creates homosexuality
  
  
  reduce resource stress on family unit, youngest child tend to be gay (E.M. Miller 2000)
  *** mitochondrial DNA carries male killing genes, with male being a dead-end (Hurst 1991)
  
  Homosexuality as an adaptive early phase of development - homosexual acts are just target practice
  
  
  homosexuality is just males practicing sex on each other, most homosexual acts are performed by bisexuals with just as high fertility
  as straight males (Baker &amp; Bellis 1995)
  
  Homosexuality advantages in other aspects of reproduction - homosexuality may imply better parenting
  
  
  homosexual genes induces feminine attributes in males such as sensitivity, empathy and kindness hereby increasing likelihood of long
  term pair bonding and paternal care (E.M. Miller 2000)
  
  


*** original text: 


  A completely different suggestion is that homosexuality may reflect
  the interests of selfish genetic elements (see 2.4.3.2) inherited
  maternally on mitochondrial DNA for which males are effectively a dead
  end. Some insects have been shown to carry 'male-killing' genes on
  their mitochondrial DNA, so perhaps homosexuality is a similar, if
  more benign, rout to sidelining males (Hurst 1991)