I tell him/her, however at the end of the day I trust my clients to know what is best for them. It is not my place to judge or to discredit their views on style or aesthetics (see my answer about 'how judge beauty').
At the end of the day my job is to facilitate their dreams in the best possible way. They pay me for my thoughts and I give them my honest opinion.
If I find that our views are not in harmony or that I am not the best person to deliver their expectations, then I say so and we respectfully part ways.
I would never set my name to something that was indecent or in violation with my core values of life and people. Respect is key.