Towards the end of Elif Şafak's The Forty Rules of Love, specifically the book-within-a-book Sweet Blasphemy which is largely a retelling of the real-life story of Shams and Rumi, Shams gets married to Rumi's student and foster-daughter Kimya. She's very much in love with him, as we see from the chapters written from her point of view. The wedding night is described from Shams's point of view, and he feels desire for her too, but he refuses to sleep with her as her husband:
I kissed her again. The warmth of her lips sent waves of desire across my entire body. She smelled of jasmine and wildflowers. Stretching out beside her, I inhaled her smell and touched her breasts, so small and firm. All I wanted was to enter her and get lost inside her. She offered herself to me the way a rosebud opens to the rain.
I pulled away. “I’m sorry, Kimya. I can’t do this.”
He steadfastly refuses to sleep with her throughout their marriage, even when she tries all the tricks in the book to seduce him. Why?
I don't recall anything about Shams taking an oath of celibacy. Nor can it be anything to do with his being a Sufi, because in Islam there's no such thing as clerical celibates or 'monasticism' in the typical Christian sense. He talks so much about love, so why does he refuse to make love with this girl, despite the fact that she wants to, at least part of him wants to, AND they're married?