What would you do??

How would you react to the following situation:

You are a young man who just got Girlfriend for about six months.
You are living with your dad and your stepmom (dad remarried after mom died of cancer).
You decide to take you Girlfriend home with you. Stepmom is there, but dad is out for a seminar for his work.
you take your girlfriend to your room but when you pass the bathroom she says she wants to freshen up.
We are 15 minutes later and your girlfriend still hasn't come to your room. You decide to go too the bathroom to see what takes her so long. When you enter you walk in on your girlfriend eating out your stepmoms pussy.

How do you react??