It isn't that it has estrogen in it, but rather that soy acts as a (rather weak) estrogen in the body. This is, soy contains phytoestrogens.
Soy protein is not the best option for those wanting to gain muscle mass.
Soy has a biological value of 74 I think. Egg is 100. Whey is higher than 100.
But I'm no expert on the BV thing so don't quote me on that.