I'm not yet a medical student (finishing my MPH and applying), but I'm in my late 20s and will never have children, either. Years of chemo during my teens and an ablation at 21 saw to that
But it's not something that I ever wanted, either. I'm too selfish. I want to be a surgeon, I want to be able to ride and compete my show jumpers, I want to be able to get in my little plane and take off for a weekend without having to worry about a family (benefits of being bored in your 20s and deciding to get your private pilot's licence...), and I want to be able to go where I need to for work without worrying that my career path is going to affect someone else. I don't ever want to be married either, and I see nothing wrong with this.