You should put your foot down harder on him. You don't like the game, yet you're willing to sit next to him and watch him play?

OKay personally, I enjoy playing DotA very much, and I genuinely do, not because some guy pressure me into it. But I also know that not all girls enjoy it. And when I go to Cyber Cafes, I often see girls sitting next to their BFs, watching blindly as he plays, obviously she was bored. And I always think... What the hell? If you don't like it, at least go surf the net or something. I always think it's stupid for girls to blindly watch their BFs play... Cuz it's damn boring to watch other people play DotA wan k, especially if you're not part of the game...!

As for your case, you should really put your foot down and tell him how you feel. If he refused to listen still, then it's okay to walk out.