Isn't the issue not what the songs say but the reason that you are singing them? I now hat sounds a bit social worker but bear with me!
The BNP story above is a case in point: once you know why they are being sung then you change your mind about singing them. If you are singing it because it is a good song, or it is evocative of a time past, or such like then IMHO that's okay. Perhaps when context changes so may judgement?