Future housemate slept with guy I like - am I being unfair?

I'm at university and in my group of friends, there are 3 girls and loads of guys. For next year, the guys didn't want to live with us girls, so instead we found 3 other people in the same position as us and deided to get a house together with them. One of them, Stephanie, has had trouble with her flatmates as they've been bitching about her, but I don't know why they don't like her exactly. Because of this, I was a bit wary of her, and she's been very eager with us, she comes round to our flat unexpectedly and so on, but I always thought she was a nice enough girl. I really like one of our guy mates, John, ever since we met in september, but he hasn't shown any interest and I've tried to move on with another guy, but i told her how I liked john. We decided to invite her out with us one night, and i left with the guy I mentioned. The next morning, I was told she'd gotten with John that night and had gone home with him. I was a bit hurt because she knew I liked him, and I thought it was a bit unfair to me if she wanted to live with me next year, that she didn't really consider how I would feel about her sleeping with him. I'm not unreasonable and I know I left with another guy and that I don't own john, but I an't help but feel she was in the wrong. Especially considering John doesn't ever do that sort of thing and I was told that it was her who came onto him. Whats worse is that she preferred another of our guy mates who turned her down, so she went for John instead. I also think it was a bit unfair of her to sleep with one of our guy mates when it was the first night we were going out together. Surely she should have been trying to "bond" with us and trying to give us a good impression? Am I being unfair because I'm hurt or was she in the wrong?