Why is it believed that, as the number of older brothers a man has increases, so does the ...

Why is it believed that, as the number of older brothers a man has increases, so does the probability of that man being gay?
 
  a. Older brothers are very likely to tease younger brothers, thus increasing the younger brother's sensitivity, and likelihood of being gay.
  b. According to sociobiological theory, it is ideal for mankind to have an approximate 1:1 ratio of men to women, to ensure survival of the species, thus as a family has more boys than girls, they will unknowingly treat the younger boys more like girls to even out the ratio.
  c. A mother's immune system becomes sensitized to the proteins of a male fetus, affecting the prenatal brain development of subsequent male fetuses.
  d. During subsequent pregnancies, the mother's placenta is more likely to be nutrient deficient, thus leading to changes in prenatal brain development with subsequent pregnancies.