If they don't do it for pleasure, what makes them do it? I think all the barbs and stuff are just biological ways to make sure that even if the animal isn't very skillful, it can still make a baby. Animals do everything by feeling - they eat when they're hungry and stop eating when they are full. They have sex when they're "horny" and don't when they are not. Also, I bet there are some things humans do for "pleasure" that look painful to most people as well.
But, then again, who knows what really goes on in an animals head....