I consider the phrase "lighten up" patronizing and would avoid it altogether. I was once told to "lighten up" by an interviewer at a bank and immediately knew that I did not want to work there. It's insulting to suggest that a person ought to have feelings other than the ones they have. In my view, feelings (not behavior) are generally involuntary, largely beyond conscious control and lie outside the moral sphere.

I'm in academia, where the use-mention distinction isn't recognized where sexual innuendo is concerned. To be on the safe side, I take the attitude that even non-interacting, space-like separated particles at opposite ends of the universe could be accused of sexually harassing each other--and probably are.