[TOMT] [Short Story] Old-ish sci-fi story with a sad sack main character.

His wife is having an affair with their son (!). Forget the rest, but something happens to him and at the very end of the story, he's lying dying in a hospital bed and his wife and son are visiting, and they push him to the side of the bed so that they can have sex on it. I remember one line from this point of the story that has stayed with me over the decades:

"In, out, in out. It's all God, isn't it?" the dying man say. "I am God. I am the Son," says the son.

That's actually all paraphrased, don't remember the exact words.

I wouldn't be surprised if this turns out to be Harlan Ellison.

Thanks.