I wonder how painful this would be compared to punching a man's chest
woman here to confirm that it is indeed very painful! for men (without developed breast tissue, obviously) it would be just punching straight muscle and fat-- but when you punch developed breasts, you're hitting a lot of sensitive ducts, blood vessels, glands, etc. that very much can rupture which is about as fun as it sounds
fun fact! males can develop fully functional breasts and produce breast milk totally naturally! some anti-psychotic medications are known to cause lactation in either sex
