Tang Beixuan might not be Tang Lixue's real son but he was Zhang Wuying's biological son. He was a product of a one-night stand she purposely had to spite her estranged husband. She knew what she did back then was wrong, but she didn't have the heart to abort her own child even if it meant that her reputation would be forever sullied by her mistake.
When Tang Lixue found out about her pregnancy, he only looked at her with disgust and turned his back on her, not saying anything. Zhang Wuying would have liked it if he talked to her, verbally spite her for what she had done. Any reaction would have been fine, but his silence was something she couldn't understand.
Since then, she and Tang Lixue never talked about the child she gave birth to. Even when she bestowed the Tang surname on her son, Tang Lixue and the other Tang family members only gave her a pathetic look, shaking their heads at her absurdity.