A Real Surprise

I'd met a woman and we started dating. After about a month, it was arranged we would get together with her parents, who I had never met.
When we got there, her mom was home, and her dad was on the way. I got along great with her mother right off.
We heard her dad drive in, and get out of his truck. He came in the door, our eyes met, and he went a bit white. He and I had been sucking each other off for a couple years, and he fucked me regularly.
After his initial shock, we just acted like we were meeting for the first time. The rest of the evening went fine.
She and I continued dating, and her dad and I continued our sexual relationship. It seemed more rewarding to be eating the same sperm that had made my girlfriend.
Of course neither she nor her mother know what we do.