User blog comment:B214/Volume 22 spoilers/@comment-73.46.6.122-20160819203110/@comment-1359322-20160822014421

That's the reason why they don't allow Harem for a Maou. They want to avoid political marriages.

For example, if Sirzechs marries another lady from the 72 Pillars let say Barbatos. Then could use the fact that Sirzechs married their daughter as a method to gain political advantage over certain issues.