What this tenant is doing is perfectly safe and perfectly legal. Imagine if the home visit discovered a huge collection of assorted dildos and the owners sought to ban their use as they thought that the toys, or he, might be dangerous. The owners are looking to restrict his private life for no good reason. Or what if the owners decided that no tenant may keep or consume alcohol on the premises, or smoke there? When do such intrusions cross a line?
Just out of interest, as dogs have been mentioned, the tenant is barred from keeping a dog in his flat. A couple of floors up, the resident has apparently bought the leasehold on his flat and has a couple of dogs.
"I do not agree with what you say, but I'll defend to the death my right to be offended by it."