I've started responding with, "Do you drink (cow's) milk?"
If their answer is yes, "Do you think that is gross?"
If the answer to that is no... "well, consider that in order to get that milk, the cow is fed with antibiotic laced feed, which often contains ground up bits of other cows (true in the US). Then that cow is hooked up to a machine that pumps that milk into a vat. From the vat, the milk is pumped into a truck and transported to a dairy processing plant, transferred into a holding tank, run through tubing, pasteurized by heating, and then transferred into containers that may or may not be completely sealed, then transferred to a store where you buy it.... I see alot of chances for contamination there. At least I know where my child's milk has been." and if that doesn't get them, I make the point that we are the only species that routinely takes the milk of another species and drinks it into adulthood.