In addition to the already mentioned Sakuta from Bunny Girl and Rentaro from 100GF, there are a couple others that I like:

• Bravern - This one is two guys that are both strong-willed. Not really sure you can call this strictly a romance though considering all the robots and apocalypse.
• Endo and Kobayashi - This is actually two romances in one. One of them (in the game world) has an assertive male character. The other (in our world) a bit less so, but it was a great series that got majorly overlooked.
• Girlfriend, Girlfriend - You can call Naoya many things, but timid is not one of them.
• Love After World Domination - A criminally underwatched series that features the Red Ranger character falling in love at first sight, and he will strive to overcome all obstacles to make it happen.
• Tomo-chan Is a Girl - Jun might be dense, but he is a man of action.