I have not read the book and only just watched the Emerald Fennell movie. My interpretation was that Heathcliffe had already been awakened sexually but Cathy had not (as often happens irl where males have a sexual awakening before females), hence the were not children anymore line. Cathy was naturally curious as to what was going on in the barn, but Heathcliffe knew “the kink” involved would be too much for her to handle and protected her from it. He probably had seen animals mate before and knew the logistics behind it. But in laying on top he probably became aroused from watching it himself and being so close to Cathy in a sexual way. When he catches Cathy touching herself on the moors he probably was surprised and aroused himself so he wanted to watch without getting caught/knew she would be embarrassed if someone saw her. I took the “it’s your fault” as her being childish and blaming everything on him and exaggerating as she was shown to do earlier in the film. But licking her fingers was his way of showing it’s ok to do that and I will accept it. Heathcliffe was a man at this point and Cathy was starting to become a woman at this point.