According to the androgen theory, the likelihood of a male being gay is increased as the number of his older brothers increases because

I believe the answer is: as the number of male offspring increases, the mother's body responds to male androgens with antibodies that cross the placental barrier

even though there is no concrete proof for this theory, This theory believes that the number of male androgen in our body would influence the homosexual tendencies that we have.
As the mother's body responds to male androgen, the number of testosterone that exist within the youngest son tend to be decrease, which tend to make that son more prone to 'feminine behavior.