If the song is sung because it is a good song, then it's fine, provided the audience are not likely to be offended.

If the song is sung to deliberately cause offence, it is irrelevant what the subject matter of the song is - it is offensive to sing it!

An acquaintance of mine used to sing an extremely funny song about Robin Hood, where the verses deliberately did not rhyme:

The friar's name was Tuck
And he did not give a damn...

For some strange reason, he thought it was funnier to put the rhymes back in. He just wanted to cause offence.

It's our duty as performers to make the audience feel comfortable - the whole essence of entertainment. There are perverse people in the world who enjoy being offended, and people who are able to tread the fine line between entertaining such people and not being at all entertaining to anyone!!