My partner has an interest in certain kinds of porn that doesn't reflect our sex life. I'm concerned he's doing it with someone else since it's not happening in our bedroom.
I’m not sure there’s a correlation or connection to this.
Let’s flip it around: Your partner is entertained by violent movies where people are killed in gory ways. Does that mean he wants to be a murderer? Probably not. (Fingers crossed for you, anyway.) It’s entertainment.
Interestingly, I note that you don’t say it’s about hotter people with better bodies than you, so I don’t suspect that’s the issue. It sounds like this is more of a kink or fetish thing. If that’s the case, it could just be that he likes seeing it but doesn’t want to participate in it. Has he ever asked you to try the things he watches? If not, then it’s obviously something he likes seeing and not doing. There’s no issue here.
If he’s asked you and you’ve said no, well, then that’s a different story. At the same time, if he’s not pressuring you over and over again to do it, let him get his jollies seeing other people doing it and continue on with your sex life. If he’s not communicating, that’s on him.
If your concern is that he does want to do that stuff and might look to someone who’s more open minded to give it a spin (visual intended), it sounds like you’re a little insecure in the relationship. That, my friend, is an issue with your communication, not his.
If, however, what he’s consuming (not necessarily pun intended, but go for it since it’s there), is something you’re willing to try, you might actually score some points by fulfilling his fantasy and surprising him. Nothing wrong with trying something new if you might like it, too.
In the end (definitely pun intended), it sounds like you’re reading too much into something. Just as we can’t judge a book by its cover, we can’t draw conclusions based on what people watch. (Unless it’s animal or kid stuff, then that’s hella nasty.)