I asked a couple of them, and they both prefer older lovers. So you can squeal about sample size and skewed research all you want, but the fact is, guys can have diversity too. Fortified with this knowledge, you can now give up the current beau and start searching for someone who appreciates maturity. In the meantime, you might amuse yourself with a study of your current lover's sexual preferences. Ask him why he likes teen-porn sites.
Fantasy provides escape from nasty real-life powerplays that can feel quite irresistible.
By imagining her, he might be reminding himself of another time, when sex felt like more but meant much less. Not everyone fantasizes about Tobey Maguire and Sara Gilbert. But if you are determined to keep him, then look for a way to understand his feelings. And by the way, you're not so old that you can't recall what it was like to be a teenager. Remember what it was for you and tell him about it.