Debora, the Church may not teach about piercing per se, but it does teach about modesty and pride.
Consider when piercings become a spectacle to others and at that point one is probably being immodest. This falls within the context of the culture and age in which one lives and certainly some cultures will view more peircings as normal while other cultures will view any peircing as abnormal. A cloistered monk peircing himself is not modesty but a nose ring for someone who lives in Greenwich Villiage in New York City may not be immodest at all. In any culture, when one begins to become a spectacle to others, they are not being modest.
Consider what one puts into the piercings as well - accentuating beauty and drawing the eye toward a beautiful face is not a sin. When one has jewelry that says, “Hey everyone, look at me!!!”, that’s nothing but pride.
Piercing yourself to elicit a sexual response from those who look at you (as some do) is certainly not modesty and intentionally inciting lust in another is a sin.
Look around and you will see that most poeple’s common sense is no sense at all. Common sense is hardly the judge of morality. There is an objective moral standard and in the case of peircings, it is modesty.
-Tim-