Lost Control Rehabing Father-inlaw In The Shower

My father-inlaw had a hip replacement so I was helping him rehab. Helping him in the shower I was surprised at how quickly his penis responded to I'm not sure what. I took it as a compliment became anxious to wash him. Teasingly I asked "what happened?" as I washed his balls, shaft, and head clean. Too stiff, hard, and big for an old man 44. Enjoying our time so much that I had to give him a happy ending. It would not have been fair to tease and not please. Now he wants it again and I tell him it was a 'just happened' one time thing and his son would not approve. My mind also thinks it's safe sex and it's just a bj. It's not as though we had intercourse. I confess I am not sure what to do and how I fell for him. I do know he needs me to help him shower until he's healed.