Do Big Brothers Make You Gay?

I read this article on Slate that mentions a study that found that fraternal birth order influences the likelihood of being gay:

In men, sexual orientation correlates with an individual’s number of older brothers, each additional older brother increasing the odds of homosexuality by approximately 33%. It has been hypothesized that this fraternal birth order effect reflects the progressive immunization of some mothers to Y-linked minor histocompatibility antigens (H-Y antigens) by each succeeding male fetus and the concomitantly increasing effects of such maternal immunization on the future sexual orientation of each succeeding male fetus.

I knew that the presence of certain environmental hormones in the womb could influence fetal brain development that produced gays and lesbians, but this was the first I'd seen of the birth order correlation. If true, the more older brothers you have the more likely it is you'll be gay.

The irony is that families who belong to religions that encourage large families -- Catholics and Mormons -- are more likely to produce gay younger brothers.

Perhaps this is why so many Catholic priests are gay (from an article in the Washington Times):

Based on his 17 years in the priesthood, [Rev. James Haley] estimates that 60 percent of the Diocese of Arlington’s 127 diocesan priests are homosexuals, which is high compared with national estimates of 30 percent to 50 percent from other authorities on the priesthood.

I guess this worked out for medieval Europe with its laws of primogeniture, which allowed only the firstborn son to inherit, leaving the younger sons with no options except to join the Church.

But it must be frustrating for these groups -- the more they do what "they're supposed to do" (have big families) the more negative feedback they get (bearing gay sons). Is it God's will? Karma? Or just biology?