Please back up your assertions with facts.
Let's start with some general observations. This study involves an unusually large sample: almost 1000 subjects. The author had some archival data from other samples, but then went to the time and trouble to collect a lot more data. Data collection and analysis is generally the most time consuming part of research. This tells you that this is not someone looking to do "quick and dirty" research.
The author analyzed his findings in a very methodical manner. Contrary to what his critic here claims, even a cursory glance at his paper shows that he neither started off presuming that homosexuality has a biological origin nor did he conclude that he had proven it. He starts off by noting previous evidence and concludes that his data are supportive. Big difference.
Let's note also that this paper was published in the Proceedings of the National Academy of Sciences. It's not known for publishing just any old tripe that someone churns out.
In my experience, anyone who would call a paper with the above characteristics "rubbish" and "drivel" is the one has a preconceived position.
Now let's turn to specifics. The critic here complains that the author has not taken into account a myriad of possible differences in child rearing practices or family characteristics. (He offers no explanation as to how some of the characteristics he reels off, such as a change in the family's financial situation, could contribute to sexual orientation.) But the important point is that his assertion is false. Although the study design does not control for every variable he manages to come up with it does control for social environment in a compelling way.
The study includes groups of men with varying sibling characteristics. Among them are men who actually had older biological brothers but were not raised with them. In other words, these men were raised as if they were the oldest boy in the family. There were also men who were raised with older nonbiological brothers.
If upbringing (family environment) has a compelling impact on sexual orientation, one would predict that the incidence of homosexuality among males who had older brothers but were not raised with them (i.e. were raised as if they were the oldest) would not differ from the incidence among males who were biologically the eldest male in the family. But that's not what the data showed. What predicted the incidence of homosexuality in this group was having older brothers regardless of whether they grew up together.
Likewise, if the familial environment is so compelling, one would predict a similar incidence of homosexuality among males who were raised with nonbiological older brothers as those who were raised with biological older brothers. But again, that is not what the analysis found. Only biological older brothers were predictive, regardless of whether the men were raised with them.
Both of these findings are consistent with an effect of a prenatal factor. It does not prove it, nor does the author claim that he has proved it.
If you read the critic's piece at the link above and compare it to the actual paper (which is linked in his piece) it will be evident to you who is operating with a demand for a preconceived outcome and who is simply following the data wherever they lead.