Humans are incredibly resourceful and will utilize just about any food source they can devise. When we domesticated animals, it didn't take long to realize we could exploit their milk for our own consumption. Don't forget, it is also used to make cheese, which is an important food source.
It makes sense, since animals can eat grass (which we can not) and then turn it into milk (which we can consume).
There are some people who also drink an animal's blood (without killing it). They just pop a little hole in its jugular, fill up a mug and patch them back up.
There are also examples of humans breastfeeding other animals, and there are also lots of reported cases of animals breastfeeding other species (dogs breastfeeding cats for instance). I have even heard a story of a baby that was breastfed by a bear (not sure about that one).
Breast milk is pretty amazing stuff. The more the merrier I say. You seem squeamish about it, so probably do not want to here stories about daughters breast-feeding fathers in prison (because they were starving to death).
Once upon a time, breast milk was used to cure a variety of ailments, such as cuts and infections. It is still commonly used to treat pink eye!
I guess I am digressing. My point it that it is really useful stuff and doesn't hurt anybody so why not.