If you liked the Mysterious Girlfriend X manga, the Anime-Planet community thinks you'd like:
Koharu no Hibi and Mysterious Girlfriend X both feature, umm... mysterious girlfriends!? So if you found the theme interesting in one of those two manga, you could give the other one a try. Just keep in mind that unlike Urabe (who is bizarre at best), Koharu can be pretty scary and disturbing...
Hideo Aiba is anything but popular. Not only is he a high school virgin, he's also a part of the "Roman Club" – a group of guys with bizarre fetishes. But Hideo's unusual school life is changed forever when the mysterious Kurumi transfers into his class, joining the Roman Cub soon thereafter. Together, the two begin a strained and pleasurable relationship where Kurumi controls Hideo's very desires... and actions.
MGX is a toned, less mature, version of Sundome. Each manga share the aspect of the main characters having a special bond with someone. MGX's bond is a little bit more kinky while Sundome's is way more mature. MGX deals mre with younger aged kids opposed to teens, when the hormones really go wild! Though if your a fan of saliva or any other wild turn-ons, then you'll love these mangas.