I think it all depends to be honest....if you were say singing along to a song as it's playing he more than likely won't mind, but if you do something crngeworthy and sit him down and launch into a Whitney number don't be surpised or offended if he bursts out laughing.
I highly doubt a bad singing voice will make him end the relationship though, more likely he'll just tell you to stop singing before the neighbours call the RSPCA convinced a cat is being strangled, like Mr Boo does with me :-(