I've been through this.
Back in high school, I was living in France in a boarding school.
One of the guys in the next room was
very openly gay, and it made us very uncomfortable. At first, we all thought what you expect a teenager to think, and joked accordingly

It was the first close gay person (as far as I knew) I had ever met, so to say it simply, I was (actually, we all were) full of homophobic prejudgments.
It turned out to be an wonderful, albeit exuberant person very easy to live with. By the second year, we were all so close that he came to our room. A great guy.
I say give it a try. After two weeks you probably won't even think about it. Once you get over his sexual orientation, you'll realize it's not that big a deal and you'll be fine.
Maybe you'll dislike him because he turns out to be a complete idiot or prefers Windows, but it'd be a completely different story.
If he's young (you didn't mention any ages) and already openly gay, it means he's smart enough to be aware of it, and mature enough to be fine with it.
Another advice would be to have a talk with him and explain him that you're uncomfortable with it. He probably has had this talk tens of times before, so chances are he'll be happy to talk about it, and answer any question you have. He has already taken the most difficult step, now it's your turn.
Keep in mind that forty years ago, you'd probably have thought the same way about being with a black roommate (assuming you're white)
Also, by being a Mac user you are a minority too. And you're open about it, and even show it on the internet, like him. And lots of PC users have wrong opinions about Macs too, right?
BTW, if some ass***e says "Hey afultz, you like gays?!" just answer him "Oh, no, I prefer lesbians"
