It doesn't emphasize safe sex.
Porn stars inadvertently teach by example. If a semester of sex ed. can pique a student's curiosity, and send them to the Internet for more information, the visuals are another extreme, entirely.
Though the issue of safe sex in porn's a controversial one, often times porn stars don't wear condoms. Granted, most are tested for sexually transmitted diseases, but those are controlled, regulated circumstances; most sex among consenting adults in America, thankfully, isn't. That said, the health risks they take on as part of their profession probably isn't something the average person should subject themselves to without at least weighing those risks. And usually, that context isn't presented with most porn.
Casual sex without protection is reckless, bottom line. Porn just doesn't communicate this (nor the answer of a recent survey explaining that sex with condoms doesn't make sex worse).