The father clearly raised his daughter - Dad is the main thing. You could always find support and encouragement from him. And to suck his cock is just a thank-you for having him. By pulling her on his cock, her father showed how much he trusts her and that secret will be with them now. And the chick did a great job - and daddy is happy and she's even closer to him now.
For that matter, I'd rather have her in a 69 position, it'd be more interesting for both of us! By the way, she did a good job on the cock, too. Probably imagined herself as a cowgirl, and her shoes as spurs, which she used to ride the stallion! When she laid down on her side, her shoes disappeared, so the ride was over? Apparently the movie was filmed in several stages, and we naively believe that the male is so hardy and can fuck a lady for a long time and not cum!
Is a big dick?