…. when Kate opens the door for him, there might have been some sexual tension between the two of them.
I really do not think anything "unnatural" was between him and the girl.
....do not think it was sexual, but rather showed instead his more "paternal" feelings ……getting to the point where it will no longer be appropriate.
I felt this his sexual tension with Mrs. Thomas was natural considering his situation. A young man that has never "been" with a woman before, every day alone in ths house with her, while she is clearly discontented with her husband, and the story remarks that she was attractive. I think almost anyone in that circumstance would begin to develop certain feelings/desires.