If you have a lot of older brothers and feel different from them, blame your biological mother's antibodies for not making you act "manly".

In some pregnant women, the baby boy's Y-chromosome triggers the mother's immune system to create antibodies, Antibodies are produced when they sensed foreign substances invading the body. Also, NLGN4Y seems to have a role in how neurons (brain cells) connect to each other. The mother's immune response may alter the function of the fetus's brain related to sexuality, which makes him not act so manly. But firstborns are not affected by their mother's immune response, only the subsequent son because after the mother has a son, her antibodies that target the protein made by the Y-chromosome increase. So gay sons with older brothers had significantly higher anti-NLGN4Y levels. However, this study does not answer how some first-born sons are born gay and why some women are lesbians. So more research is needed.

Plus, this study does not indicate that being gay is some sort of disease. It just tells that being gay is inborn and not a choice.