I think being gay or bi makes one want to watch gay porn.
It seems to me that your partner could be bisexual and unaware of it, or secretly bisexual, or perhaps just 'curious'. Are you sure no-one else logs onto this particular computer?
(I hate to say it, but if one goes looking into another's bookmarks list, or private diary, or facebook page etc, it is a violation of that person's right to their own privacy, their interior emotional / mental realm. We all have the right to demand this private space. And eavesdroppers always seem to injure themselves gaining info they shouldn't be privy to - it can be an extremely socially hazardous act. That being said, nobaodys perfect, we all do things we oughtn't to, I'm not judging you for your follies).
Why I think he is bi: he watches gay porn but he is also going out with you.
I should add that a lot of straight guys like to watch gay porn anyway, they just won't admit it, the wussies. Maybe what you discovered on his computer is simply No Big Deal.
I gotta say, bi guys are great! The worst thing that can happen to a bi human is being forced to pick a monosexual role - gay / straight. This is why we bis are the largest invisible sexual group on earth (aside from lesbians perhaps); society forces us to choose one strict path and be less than what we can be. For many people this is unsatisfactory.
I love being AC/DC. People are great!
If you think you want to stay with this guy, and if you discover that he is indeed secretly bi, try and be supportive of him during a potentially difficult time. It will be worth it. It was for me and my bi partner, we have a greater level of mutual understanding nowadays, and heaps more fun. These crises lead to greater levels of trust and intimacy if handled well. I wish you well.