I remeber reading this i think on webnovel. Its about a guy who can move to another world. He gets married to the queen because she wanted someone who was not after her power. He also brings stuff from earth and helps to do reforms and stuff. But he can only travel to earth every 10 years or something due to alignment issues. Its not harem or andything like that just a regular romance novel.
Empress’s Gigolo I think. It was pretty good at the start, I liked the character. He was weak, but he tried his best and used modern means to get around it. Then he got some power and devolved into just another murderhobo. Quite disappointing.