These days, what is legal is often less relevant than the corporate policies of those you do business. This is a form of (legal) corporate censorship - fundamentalist political pressures applied to economic infrastructures result in dogmatic policies passed down the front lines (unprotected by constitutions).
Rather than splitting hairs on semantics re erotica/art, your CC processor clearly states the following as a violation of their TOS:
D. The posting, display, or advertising of any image using a model or models under the age of 18 years anywhere on the site whether the models are clothed or unclothed.
If there is any doubt, then you will need to prove it.
'clothed or unclothed'
-Dino