There is a big difference between “treated as porn” versus “this part of anatomy is specially evolved to be inserted into the mouth of an infant, so we don’t care if you show pictures of that exact scenario”.
I agree bans on images of nursing are absurd, but I am not sure you can make an appeal to evolution - genitals also are 'specially evolved' but also not postable in many places...
If my sentence had just been “nipples are fine because they evolved”, that would be a reasonable counter; it is what they have evolved for that makes it weird to censor them as they are censored.