using the analogy of a partner coming up to a woman and piercing her with no warning doesn't follow.
I choose my child's clothes, food, hair cut, when to brush her teeth, what activities she does, who her friends are.
If my partner did any of that to me there would be outcry.
A baby is an extension of the parents initially. We see this in the way we treat their bodily fluids. If I cut my finger I'll suck it. If my daughter cuts herself I'll kiss it better. I physically sucked the snot out of her nose when she was small and struggling to feed from congestion. I'm not going anywhere near anyone else's snot or blood. My own. And my daughter's. As she is growing up, I'm getting the normal aversion to another person's body. I can no longer bear to think to suck her snot etc.
So this lack of sense of "otherness" of a new born. I am her. She is me. May be why some parents do not see it as a big deal to make such choices for their tiny baby child.
And you can dismiss culture, but if it is normal for everyone around you to pierce a child then you are far more likely to do it.
Im currently "training" my toddler's hair so she can have a parting. This involves combing her hair and putting it in bunches. I have made a cosmetic choice for my daughter. There's a functionality to it - far easier to keep out of her eyes than with her natural whorl parting. But it is uncomfortable for her daily to have her hair brushed. And it is my choice to give her long hair because culturally that is normal for little girls. I could just cut it short out never ever comb it. Instead I choose to inflict a small amount of pain daily so that she can look nice and cared for.