I've been through this with my partner, you can feel free to message me about it privately if you wish.
To answer your question in short: yes, straight men can look at gay men but truly straight men do not usually do that. It is more likely your boyfriend is bicurious, bisexual, or "heteroflexible". If the possibility of any of these things bothers you and they are things you do not think you can reconcile with I would suggest ending the relationship and saving yourself any heartache.
Oh, and I don't really believe you stumbled on that looking for something to bookmark. That's actually the exact excuse I used to justify my own snooping and inevitable discovery.
It's confusing, yes. But honestly it's not the end of the world if you desire a long term relationship with this person. It sounds like he is afraid or unwilling to admit his curiosity, which is completely normal because there is a huge stigma attached to it. An accusing tone or an emotional outburst isn't going to make him more comfortable admitting whatever it is he is feeling so tread lightly.
Oh, and you should also expect him to start cleaning his cache.