http://ussec.org/wp-content/uploads/2015/10/SOY13_7_Soy-and-...

"In a Harvard study published last year in the journal Human Reproduction, Jorge E. Chavarro, M.D., Sc.D., and his colleagues found a strong association between men’s consumption of soy foods and decreased sperm counts."

The study in question seems to be this:

https://academic.oup.com/humrep/article/23/11/2584/2913898/S...

"The intake of 15 soy-based foods in the previous 3 months was assessed for 99 male partners of subfertile couples who presented for semen analyses to the Massachusetts General Hospital Fertility Center. ... There was an inverse association between soy food intake and sperm concentration that remained significant after accounting for age, abstinence time, body mass index, caffeine and alcohol intake and smoking. In the multivariate-adjusted analyses, men in the highest category of soy food intake had 41 million sperm/ml less than men who did not consume soy foods." (For reference, various sources say anywhere from 20 to 150 million sperm/ml is average.)

Contrariwise, the Wiki article on soy points to this meta-study (from four years before the above):

https://www.ncbi.nlm.nih.gov/pubmed/20378106

"Clinical evidence also indicates that isoflavones have no effect on sperm or semen parameters, although only three intervention studies were identified and none were longer than 3 months in duration."

Here's another from a few months after the meta-study:

https://www.ncbi.nlm.nih.gov/pubmed/19819436

That last one seems to be a randomized controlled study, where they gave a bunch of college men either milk protein or one of two variants of soy protein.

I don't know if there's anything in soy that isn't in the "soy protein isolates" from the last study. If there isn't, then the last study seems like strong evidence. The first one could conceivably come down to some sort of selection bias.