He had a FULL harem of concubines  I know ..

But he didnt come across as a horny guy who has a glad eye for the women .. especially a woman he knows is his mom's pawn piece AND and daughter of his 'enemy' who might be a plant (he allowed her to sidle along during court discussions), he also knows she went after XW's girl (he was happy to see XW having a love interest and might settle down in marriage)

Did I miss something in the plot?

Was it because he wanted to control her father ? But he knew she had cut off all links to the family .. 

You should post in the public comment, more people would see it and answer.

:-D

Thanks .. Good idea

He is trying to stop Xue family from installing Ling Zhi wang as crown prince. His mom wouldn't stop plotting with her brother xue yuan. They wanted to  make Ling Zhi wang crown prince and basically kill Shen Lang but he thought marrying Xue Shue would kill two birds with one stone. Ling Zhi wang already begged him to Marry Jiang Xue hui so he got rid of Xue Shu from the running in the consort selection and stop the mother from trying to install a crown prince.

They were using each other. She likely  had information he could use against her father. Xue Shi could only rely on the emperor anyway since she turned her back on her family at that point. Xue Shi thought she could manipulate him or worm her way into his heart but he clearly always had his guard up. The emperor saw her as a pawn and probably just found her attractive enough to play along. 

I think they're confused because Shen Lang doesn't come across as licentious man. Shen Lang does explain to Lin zi that it's to also help him marry the person he wants but of course the mother interferes but the prime reason stands that when he beds Xue Shu, Xue family can no longer install Lin zi wang as crown prince they want to install  him as the crown prince so they can control  his future baby  by killing him  the reason they're choosing this route is because  Shen Lang has become too independent and doesn't listen to them any longer. The two brother sister pair (the empress dowager and Xue Yuan) installed shen Lang and  the queen ruled as regent but Shen Lang grew up and took control himself so they want to get rid of him by forcing him to crown Shen Jie as crown prince(which is what happened in the previous life) so he beds Xue Shu to secure his seat.

I know.  It was so gross, ughhh.  Nasty!

What is even odder is how suddenly Xue Shu becomes his most trusted advisor and apparently the most powerful person in the palace.