According to the theory/premise, back in the early days of our species' development, when life was comparatively hard and extremely dangerous in the wild etc., a stressful event experienced by a pregnant mother indicated that the environment was hostile. And therefore, the environment was not condusive to mankind being born into and continuating to procreate in it. Accordingly, Mother Nature then decides to "neuter" the foetus's future sexual disposition. Thus, the baby is then born as "gay" or homosexual, with no natural urge to procreate etc. (in what Nature has deemed to be an unsuitable/hostile environment).
Now, that's only a theory - not one I would subscribe to, exclusively - but it's food for thought maybe :)