And due to different cultural norms, the Japanese in general don't view titillation as necessarily bad or reserved for adults only. Teenagers get horny and are interested in sex. Japan accepts that, while the west often pretends puberty happens all at once on your 18th birthday.
I'm not sure why you're bringing up Catholics. It's the evangelicals that usually get their panties in a twist over a nip slip. The Catholic-majority western countries I'm familiar with (mostly Spain and France) tend to take a more laid-back approach about that sort of thing.
I don't understand what point you're trying to make here. In the west we don't put nipples in magazines aimed at teenagers because that's culturally unacceptable (and often illegal). In Japan, they sometimes do. It's up to the magazine.
But it's not like they're a monolithic culture. They've got prudes, too. Some magazines choose to censor the nipples to keep the complaints from those prudes at bay, but by and large the general public doesn't have a problem with a teenager reading a mange with nipples in it.
