Anna’s older mystery man?

Towards the end we see Anna (Olivia Wilde) meet an older man and sleep with him. I read several comments and reviews mentioning that the older man is actually her father. The first night we seen them sleep together she says “daddy” as she goes to sleep but why are people interpreting this dialogue as him being her actual father as opposed to the slang/role play term “daddy”.

I don’t see how that was her actual father and why would she sleep with him. Perhaps people took the phrase too literal. There was no incest implied.