I've heard the same thing -- "humans are the only species that, in adulthood, suckle from another species."
But looking at it from a more religious perspective (as opposed to a Darwinist, scientific one):
Man has drank milk from various animals for centuries. It's one of those "natural foods" that goes back to the cradle of civilization. I doubt people drank 4 glasses a day or anything, but it was certainly used by adults.
Matthew