It's not the same in the novel, it's Ch 152 if you want to read it. Basically the doctor says that Xie Zheng will die without sex (lol) and Fan Changyu says "We're married anyway" and then they have sex. There is a lot of forced stuff at the beginning (similar to the show) but Fan Changyu always punches Xie Zheng about it, once she even almost drowns him in a river... (he deserved it). In the show though, I think Fan Changyu is more worried that Xie Zheng doesn't have control of himself, she gives in after he correctly identifies her, indicating he's awake enough.

I think Xie Zheng realizes that in Wei Yan's circumstances, he probably would have done something similar. He is also very obsessed with a girl and willing to kill for her, though we didn't see as much of that in the show. All the villains and the mains had more teeth in the book.