Gmail to explain why a particular mail is marked as spam.
Google posted on its official blog, "Starting today, we'll be showing a brief explanation at the top of each of your spam messages. Simply look at any message in your spam folder and now you can find out why it was put there and learn about any potentially harmful content within the message."
This would certainly answer your naive queries on why most of your 'adult' mails automatically land in the spam box and 'educate' you about phishing scams.
As Google puts it, "We hope that this is not only interesting, but also helps you learn about scams and other harmful messages that Gmail filters out. Whether you prefer to leave your spam folder untouched or do some educational digging, the information will be there for you. And if you're interested in learning more, check out our new series of spam articles in the Gmail help center."