My Girlfriend Sléeping with My Dad’ – Boy seeks Advise

I caught my 18 years old girlfriend making love with my Dad. My name is Peter, am 23 years old. I have been in love with my girl who is 18 years old. We have been dating for 2 years now. She was caring and loving. She told me she’s a vírgin and we had agreed to have s*x after marriage.
We were only concentrating on our studies. She could come home at any time. I stay with just my dad and two little sisters. Early this week I had taken my little sisters to my auntie’s place. When I returned home, I found my girlfriend in bed with my dad.
This was terrible and shocking but that’s not all.
That very day my dad chased me away from home. My GF never bothered to call me to say anything about it since then. But today she called me, crying that I should forgive her for what she did.
Friends I am confused cos I truly loved her but I don’t know what to do.
Is right for her to be sleeping with my father? Should I forgive her or I move on? Since the incident I’ve been staying with my auntie. Please I need helpful advise.