I just received an email asking about this. Here you go, from the BBC (note also the comment about Asian males and soy--in Asia people don't eat boca burgers, it's all fermented soy, and not usually as a main course btw):
A regular diet of even modest amounts of food containing soy may halve sperm concentrations, suggest scientists.
The study, published in the journal Human Reproduction, found 41 million fewer sperm per millilitre of semen after just one portion every two days.
The authors said plant oestrogens in foods such as tofu, soy mince or milk may interfere with hormonal signals.
However, a UK expert stressed that most men in Asia eat more soy-based products with no fertility problems.