I try to understand the evolutionary advantages of the body reactions of nipples and penises/vaginas. I'm confused, because I can explain the evolutionary advantage in a logical & consistent way for penises/vaginas, but not for nipples.
arousal causes stiffness
Penises are getting hard when aroused. I can make sense of that - it is evolutionary advantageous, because sperm is more likely to reach the uterus; therefore the likeliness to get pregnant is higher; therefore providing an evolutionary advantage.
But I can not make sense of nippels getting hard. I don't see any advantage that could provide. I mean, breast feeding causes stiffness through stimulation; but that is not sexual, I guess? I guess, breast feeding woman don't experience arousal because of the breast feeding?
Furthermore, why does nipple stimulation even cause arousal? I mean, why does it cause evolutionary advantages that we feel arousal when they are stimulated? Again, I can understand this part for penises/vaginas.