does your dad know how you feel about this? tell him he ought to communicate with your mum and explain how he is feeling. ask him what he could do to change the situation, or if its really serious they should get marriage counselling. goodness, your mum deserves respect and it would be silly for the marriage to end this way just because your dad doesn't feel his needs are being met!
but before you give any serious advice to anyone, I'd go see an adult you trust at your school, preferably one that is/ has been married. because they will give better,more trustworthy advice than strangers on the internet
I'm sorry you have to go through this, my heart goes out to you! I hope the situation improves!