That’s the million dollar question. Some of the novels expand on the telepathic bond that develops between mates, a la T’Pol and Trip in Enterprise. I think we can probably assume that it would also exist between Vulcan Sarek and human Amanda. Sarek is a passionate man, underneath the facade, and he is also more affectionate with his wife in a private setting in Journey to Babel.
Amanda would also have had to deal with an out of control Sarek during pon farr. I wonder if they met when he was on earth through the Vulcan embassy and became friendly and formed a bond. Then, she was the only suitable female around when he went into an out of phase pon farr triggered by being off planet. She not only handled it and him, but enjoyed the experience enough to marry him and have his child. It’s apparently possible in Star Trek for a Vulcan and human (or Romulan and human) to conceive naturally, though there are more difficulties. I would guess they fostered Michael because It was too dangerous for Amanda to have a second child and they wanted a daughter.