My take from Selfish Reasons to Have More Kids:


Parents
have a small effect on sexual orientation.
Psychologists used to
label homosexuality a “mental illness” caused by overprotective mothers and
distant fathers.[i]  Now we tend to see sexual orientation as a
preference inherent in our genes.  When
you look at the actual evidence from twin and adoption studies, however,
neither story quite works.  Genes
definitely play a strong role – every major twin study finds that identical
twins are more alike in their sexual orientation than fraternal twins.[ii]  Yet genes are far from the whole story – if
you’re gay, your identical twin is usually
still straight.  Furthermore, family
environment clearly affects sexual orientation. 
The smoking gun: Adoptive brothers of gay men and adoptive sisters of
gay women are about six times more
likely to be gay that you would expect from chance.[iii]  On balance, the evidence that parents make
kids gay is a little stronger than evidence that parents make kids
well-educated, rich, pious, or chaste. 
While heredity is the star of the story, upbringing plays a modest
supporting role.


[i] Bayer, Ronald.  1981.  Homosexuality and American Psychiatry: The
Politics of Diagnosis.
  NY: Basic
Books.

[ii] “Genetic and Environmental Influences on Sexual
Orientation,” in Handbook of Behavior
Genetics
, p.271.

[iii] “Genetic and Environmental Influences on Sexual Orientation,”
in Handbook of Behavior Genetics, p.271.