Jesus how could he even think of s$% with ET, while poor Arthur was stood at the bottom of the stairs half dead. He is the lowest of the low. Too busy thinking of himself , no thoughts spared for Arthur.
I don’t think them having sex relates to the period of time where Arthur was gravely injured. By all accounts, TH was out when that happened and came home and started CPR. I believe in general, Ms Prior is stating that they would have sex whilst Arthur was awake downstairs.