I'm thinking of opting not to have children with my girlfriend that has inheritable autoimmune disease
Does her disease concern you a lot?
It doesn't concern me that much.
While children are expected in a relationship it is okay not to have children. Responsible people have to think of all the possibilities when having children since those health concerns can affect them and your future.
I'm content with my decision to not have children.
If you are happy with it and you have discussed it with her than that is all the matters.