I'm thinking of leaving my parents house during a horrible time
What do you plan on doing when you leave?
No. I'm thinking of starting a family with my girlfriend
Spending time with the family during the holidays is difficult and it is understandable if you have to leave. You shouldn't have a bad time just because it is family.
I would just rather spend time with my girlfriend instead of them.
It might be better to leave than keep the tension during the visit. It is great that you are planning on starting a family and having kids. Your parents might want to be involved with their lives when they are born.