Is it ethical to withdraw an accepted manuscript for financial reasons? I submitted a manuscript, which has been accepted. Its publication is pending payment of publication fees. I need to withdraw the manuscript due to financial constraints.

Is this ethical?
The fundamental ethical principle is that you shouldn't waste people's time.  This could be applied differently, depending on the circumstances:


If you knew there were mandatory fees you couldn't afford, then it was unethical to submit the paper.  The damage is already done (assuming you really can't pay the fees), and now you should apologize and withdraw the paper.
If the fees were clearly announced but you just weren't paying attention, then you screwed up.  Again you should apologize, and you may have no choice but to withdraw the paper, but it's worth asking whether fee waivers are available.
If you submitted the paper in good faith, believing that you would be able to pay the fees, but your financial circumstances have since changed in a way you couldn't have predicted, then you should explain the situation.  You may need to withdraw the paper if you can't get a fee waiver, but you haven't done anything wrong.
If the fees were not announced ahead of time (for example, on the journal's web site), then the journal is at fault.  In this case, you should complain and withdraw your paper, since you don't want the paper to appear in an unethical journal.  However, you should check very carefully to make sure the fees really weren't disclosed, since it would be humiliating to accuse the journal of fraud when you just weren't reading carefully.