If I am understanding it right, the guy wants to introduce her to his family, so he does. And his dad seems to have NTRed the girl...ok...why? She starts "feeling it" on like page 3 (she says something like it feels like he is staring at her naked body through her clothes, and that seems to excite her) she gives herself up to him with literally no resistance, one page 6 he asks her if having sex with the guy isn't satisfying her, and she replies "As I thought, he can see right through me"....[so I guess they've had a relationship since before then] -->I was wrong on this assumption it seems, she's just a really fucking "easy" girl.
@redbone thanks...(I guess, I hate NTR, so seeing the English doesn't really make me any happier. If anything, its negative for me. But thanks anyways) @rexz you're absolutely right. She was ready even before she had sex with the dad