You would not be the first person to do it. The problem is that you wind up with a lot of heartache over it. Think twice, but in the end it is up to you.
It's your own personal decision, but I would consider it a foolish, immature thing to do. That sounds harsh I know, but why set yourself up for heartache and frustration. Move on now instead of waiting till later.
Not if you are absolutely positive you can't be with them, but you can't help who you fall for. So just let nature take course.