That's the problem. I can't change the setting in Search, despite having already changed my settings to General, Moderate, Adult in Preferences.
Also, in my naivety I submitted a support ticket and was told "Our records show that your Second Life account has a Basic (free) membership. Basic members have many Support options open to them (you can file cases about most account issues, billing issues, marketplace issues, and some technical issues), but it seems like your issue in this case is not one that is supported for Basic members."
Perhaps that was meant to encourage me to become a Premium Member but believe me, it had the opposite effect.