I think that the point behind Milk being an objectionable substance is not that it is poisonous but because farms treat the cows badly when they farm them. Though I do think that milk farming is less objectionable in than the practices used for farming other animals. It is not that the meat is bad or the milk is bad ... it is that under current farming practices with big business making big bucks ofen leads to the inhumane and downright cruel treatment of animals. If you want to eat animals then fine but I don't think we need to torture them first and being a vegetarian means that I am no longer condoning the torture of animals just as I wouldn't want any part in the torture of human beings.
If you want the facts as to why peta objects to milk then have a look at:
http://www.goveg.com/rfact8.html