A bike boner is what a cyclist gets when they see a very nice bicycle either on the road, in a shop or locked to a bike rack.
In most cases it is a bicycle that they themselves cannot afford.
A hipster would see a fixie all decked out in radical ironic colors and totally pop a bike boner.
A Performance Cyclist would see a Trek or Cervelo decked out with all the most unnecessarily expensive components and get a raging spandex bike boner.
A Cross-Country cyclist would see panniers, saddlebags and fenders and erect a towering bike boner.