It's not even a question of ethics, actually.
I'm working for my employer for a living. No matter how many (s)he's paying me, it will **never** be enough for me to accept to go to jail. And that's what is very likely to happen if I accept to pay a bribe.
So it's just not going to happen. No matter what the consequences could be, for example being fired, I'm not going to let myself getting involved in illegal activities for my employer.