Ok, so this is not really that clear-cut. That sounds like a pretty generic porn concept to be honest. If he is specifically looking up videos featuring sex with a roommate, then he might have a passing fantasy of sex with a roommate, and yeah there's a good chance he might have imagined (but not seriously considered) sex with your roommate. Most committed guys will occasionally fantasize or imagine sex with miscellaneous girls in their lives. In many cases, you fantasize about something because it is forbidden and you know it would be "bad" in real life. Porn featuring sex with a roommate is common because everyone logically knows it is a bad idea to sleep with your roommate, but the idea of doing something stupid and compromising a close relationship out of lust is a pretty typical fantasy.

I've had dreams and imagined sexual situations with several of my gf's friends and roommates, usually in the context of a threesome with my gf. This stuff pops into my head almost involuntarily, and I would absolutely never consider the possibility of doing it unless my gf desperately wanted to. Even if she did, I would push pretty hard against the idea because I logically know it would seriously complicate our relationship and I care more about our relationship than indulging in one passing fantasy. Still, I might occasionally look up porn about threesomes with a gf and her friend just to get the fantasy out of my system. After all, it isn't sex with any of my gf's friends in particular that I am fantasizing about; it is the idea of sex with my gf and one of her friends. Your bf is likely in a similar boat with roommates.

You can't and shouldn't try to police people's fantasies, but it makes sense that this bothers you. Your bf probably doesn't want you to find out and might have feelings of guilt about even imagining this. You could talk to him about it, but be careful and try to approach it with an open mind at first. You don't know what is really going on in his head. If you seem overly worried or hostile from the beginning, he might take that as a sign that he should bury his secrets and desires from you in the future. If this roommate thing is a fantasy of his, one possibility is that you could try indulging it by doing some roleplay where you act like you're just his roommate. In any case, I would suggest approaching the topic in a positive/neutral way initially like "Wow is this what you're into?" Afterward, you can ask if there is something else going on and tell him honestly that this worries you. You really need to know what is going on in his head, and hopefully he'll be willing to talk to you about the topic. It might be good for you to tell him about some kind of secret fantasy too. If everything else is good with your relationship and he is willing to talk about it, you should definitely be able to work through this. If he becomes really evasive or is afraid of discussing the subject, then you might have a bigger reason to worry.