One condition of him having a phone was that we know the pass code and can look at it to make sure he's staying safe online, which he's happy with. TBH we've not looked at it that much as it tends to just be WhatsApp conversations of total drivel. 
But this morning a new email came up on his locked screen saying something along the lines of 'you've recieved a new flirt from fling.com' so I looked at his phone out of interest. He's set up a separate email account (he has one we've set up so he can email family, etc) and all the emails in there are from this fling/flirt site, and others, one called fuckbook. 
He doesn't have access to a credit/ debit card so he can't have paid to subscribe to these, so I'm assuming these are free versions. The one email i opened had a photo of a woman/girl in her underwear.
He's not showed any interest in girls so far, in fact he seems a bit young for his age compared to some of his peers. So I was quite shocked to see these emails.
DP thinks we should just ignore it, but I feel uncomfortable about it. Am I being naive? Is this what teenage boys do??