There have not been many studies into this, and this particular study is but the third to be completed. Nevertheless, the idea that there is a causal relationship between the eating of soy and the reduction in sperm is an interesting one. I need to get a copy of the study and have a look at who the subjects were. Indonesians for example eat a lot of soy-based products. Since I have been living here in Indonesia, soy has always been a staple and has always been present in just about every meal that I have had. I love the Indonesian forms of soy and regularly eat tahu and tempe.
It seems that soy contains isoflavones, an organic compound which acts like female hormones, which then appears to impact negatively on a man's ability to produce sperm. The science is that the isoflavones in soy are structurally similar in the chemical sense to estrogen and then serve to mimic the way estrogen acts in the human body.
The study involved 99 test subjects all who were involved in a relationship where there were difficulties in conceiving. There was an "inverse" association between soy intake and sperm concentration. This inverse association was distinguishable even after other factors such as age, abstinence, caffeine, BMI, and alcohol were taken out of the equation. The average man has a sperm count of between 80 and 120 million per millilitre whereas the soy consuming subjects had an average sperm count of just 41 million per millilitre.
I guess the next time I am contemplating getting into the baby making game then it is time to swear off the soy.