Some men seem to think a bite of tofu will cause their balls to fall off, since soy contains estrogen-like isoflavones.
A major review of 15+ studies on this subject concluded that eating soy products has no effect on testosterone levels in men [Hamilton-Reeves 2010].
However, this review also included research on elderly men with prostate cancer.
I took a scrutinizing look at the studies done in young healthy men, and left out any rigged data. The image shows the results.
To put things into perspective, you can see what adding 30 g of fat to your diet might do for your testosterone levels [Volek 1997].
In the worst case scenario eating more than 300 g of tofu per day will cause a trivial decrease in testosterone that fades compared to the effects of more fat in your diet.
Here’s an overview of the studies included in the image.