It depends. There is a range of approaches. The classic approach, of which I think you are speaking, is indeed very short term. Girls are paid per day/shoot/scene and that's all. Other girls own and operate their own websites, retaining copyrights, and can generate a stream of income over a great many years. There are women out there in their late twenties still releasing "new" material shot while they were 18. Many who opt for this approach don't do sex, or even nudity, and so don't suffer any health repercussions. But that takes planning and a degree of time/money investment.

I wouldn't encourage the career, but I would fault any teacher ready to dismiss the industry. If you ask kids what they want to do, you shouldn't be critical of answers that conflict with your personal morality.