I second Bitter Virgin. Great manga that's not appreciated enough, with a very interesting premise.
Also, if you're looking for something with much more comedy, and if you're ok with a small amount of occasional near-erotic moments and reality-testing plot devices, you could try B-Shock. Two college students who'd never really met and are as different as can be are abducted by an eccentric and infamous professor, who decided to select them for an odd "experiment". Both are forced to wear bracelets that can detect the distance between them, and that contain explosives sufficient to pretty much guarantee fatality. The nature of this romance is complicated, since even after feelings start to form, they have to wonder whether it's actual love or something more on the lines of desperation since they can't possibly pursue anyone else, or even be alone.