And why would the grandfather resist? When else would he have such an opportunity. Besides, it's always easier with a temperamental woman - she gets herself turned on and therefore comes much faster. You would have tried to bring to orgasm phlegmatic fat girl with a huge hollow. That's the kind of woman you should run away from. I've been in the middle of one of those, you can't wish it on the enemy.
Well judging by the way her ass took a member of her lover, it is safe to conclude that anal is not a new thing for her, so no wonder that he fucks her so bravely without slowing down.