Apparently they put a lot of value on the honor of their women back then, so a threat of donkey rape held a lot of weight. The artist even carved in that cute little sun and moon on the top, which was to say to the viewer, "This donkey's dick will be waiting for you as long as the sun and moon exist." It's very good art, is the point. Here's some with a bit less subtlety:
via New Indian Express
Any critic of animal sex threats would agree this one is a bit ham-fisted.