fraternal birth order isnt a new discovery. the problem with your post is the application of genetic determinism to explain homosexuality, while clearly having a (being charitable) high-school level understanding of immunology and physiology. The extent to which it is genetically determined (predisposed, to be more accurate) is a fairly intense field of study, but your proposed mechanism and presumed source offer incompetent and mechanistically inconclusive explanations, respectively. Further, any attempt to define homosexuality as a predominately genetic phenomena is spurious in the absence of more rigorous study. Your (presumed) source offered no variance assessments between homologs of hetero- and homosexual carriers, so arguing that the heritability* of homosexuality at all is a poor interpretation of the data I gave you. Considering you haven't posted a source yourself, I'd reccomend diving a bit deeper into the peer reviewed literature before making further claims. I'd also reccomend brushing up on comparable phenomena where maternal immunoresponse results in developmental "defects"**, e.g. rh-factor induced hemolytic disease in newborns. In doing so, you'd notice that the response is triggered not by immunoperception across the placenta, but rather by child-to-mother blood transfer occurring during delivery. In the absence of a similar exposure (e.g. one in which the fetal/neonatal nervous tissue is introduced into maternal circulation for immunorecgonition), I really look forward to hearing how you explain the maternal immune response arising to begin with. I'd like reiterate that the protein of consequence here is an embedded surface protein in fetal neuronal tissue, and not in free serum, and so there needs to be some mechanism by which fetal neuronal tissue is exposed to maternal immune cells.
*I use heritability in the rigorous sense here, cf.
** I use the term defect here cautiously. Within the broader framework of evolution I do not like the term as it carries significant negative connotation, and within this subject it risks characterizing, in light of further study demonstrating genetic causation of homosexuality, homosexuality itself as a disorder or defect i.e. something to be "corrected"