I used to have difficulty explaining to people why I was unwilling to work unpaid overtime, but I've finally settled on just saying "It's a moral issue; I think it is wrong." This seems to shut people up, at least, though they probably think I'm some kind of nuts Marxist.

Asking for more labor in return for no more pay is wrong and allowing this kind of immorality to happen by willingly participating in it is also wrong. Therefore, living a moral life is incompatible with unpaid overtime.