Why is it gross?
We routinely drink the breast milk of non-human creatures, and no one considers that to be gross.
...because there are human diseases that would get passed on to the general population, that just wouldn’t come through a cow or a goat. It’s different when a woman passes something on to just one child, her child.
It’s gross because it’s bizarre and unseemly — somewhat akin to cannibalism (with which, I am sure, the PETA ghouls would have no problem.)