The Portable Star by Isaac Asimov - Holden made love to his friends wife. Because he couldn't help it.

If space voyages are "romantic", Holden Brooks was certainly carrying on the tradition when he stepped into the cabin of his best friend's wife, with one straightforward objective in mind.

He did not signal. He merely opened the door and walked in. She was waiting for him as, somehow, he had known she would be, wearing a loose garment. She held out her arms to him and they trembled slightly. Her dark hair fell below her shoulders, accenting the pale roundness of her face.