If you're looking for manga similar to To Love Ru Darkness: Special, you might like these titles.
20 years ago, Weve Aliens arrived on Earth to search for marriage partners. Weve are perverted and love interbreeding. With their advanced technology, they have solved all the Earth's problems to do with the environment, food, and resources. Gen is a student at the prestigious Human-Weve school Shinryou Academy. Due to his late grandfather's words, he loathes Weve and believes that pure humans will eventually die out if something isn't done about Weves soon. However, Gen meets the Weve girl Mirika, who falls in love with him.
Ren Sonoda, 13 year old middle school girl, is abducted by a UFO one night and she finds she has an extra appendage between her legs in the morning! A space chicken promises to return her to normal, but only if she'll help track down an intergalactic fugitive...
Shibata Youichi is an ordinary high schooler, but one day he runs into a mysterious heroine! The cute girl knows his name, and (as in a stereotypical anime) she's an alien who needs him to help save her people! Soon enough, Youichi finds out that this situation was too good to be true... The girl, named Kyara, does represent aliens, but her people are actually interested in Japanese manga/anime/game culture. They want to use his home as a base and his brain as a measuring tool of the level of appeal of their heroines!
Junior high student Mao Hoshimi is a small and somewhat effeminate but otherwise typical boy. At least he is until he turns into a girl while hitting the baths with his best friend Shunsuke Hidaka! But this is the least of his shocks--Mao learns he is actually of alien descent! Because the numbers of his species are so few, Mao learns, its members have the ability to change shape to breed with other species--and also to transform from male to female and from female to male to suit a potential mate. It seems that, now that Mao is in puberty, he is fated to become a suitable partner for whomsoever he comes in close contact with, be it Tomoko Yokawa, the cute girl he just met, or, to his horror, Shunsuke!
Two aliens are sent to Earth to scout for resources: that's how their adventures in Tokyo begins.