I hear a lot of people saying soy beans are poisonous. what evidence are they basing this on? pesticide residues or effects of phytoestrogens?

Anti nutrients : It's more an issue of natural chemicals in the soy bean that bind nutrients. That is mitigated by fermenting the soy (miso/natto/some tofu). There are concerns that consuming large quantities can alter estrogen levels so it shouldn't be eaten daily. The last issue is gmo- most commercial soy is modified and that is not fit for consumption. The toxins in that seed alters gut flora.