Well, without repeating all the comments from numerous people who have said, basically, "grow up," I'll stick to my area of expertise: mathematics. Of course, this is 3rd grade mathematics, so if your math competence is on the same level as your ability to deal with your dad's gf, you should still be able to handle it, barely.
You're 23. If she marries your father, she's going to be your step mother for possibly 40 years. That's nearly twice as long as you are old right now. Think about it. Every time there's a family get together (xmas, etc.), she's going to be there. When you have children of your own, she is going to be their grandmother. So, quit being a dick.