If you can't justify that, then don't contribute. It's that simple.
The license wasn't chosen randomly - it's there exactly so you can't use their time and effort by porting their components to closed source projects. It's not an unintended side-effect.
If your code is in fact completely yours, and not derivative work of others, you can simply re-license. If it isn't, you can hardly claim that you were duped. Or are you really saying that anybody mislead you on the copyleft aspects of the GPL?
The license wasn't chosen randomly - it's there exactly so you can't use their time and effort by porting their components to closed source projects. It's not an unintended side-effect.