Only so many times you can watch the same young, vacuous boring sex. Exercising, cleaning or video games topless or nude can just as thrilling to watch. Even the same masturbating makes you switch and go elsewhere. Some will say it's a hard job, but not hard to try new positions/places in the apartments. Sad state when people 30/40+ years have better and more adventurous sex lives.
Also, think if you get the right people you can have tenants geting on with their lives, without it looking like a performance all the time. Employ older.