Three's Company/WMG

Mr. Furley knew Jack wasn't gay from the "Baby, It's Cold Inside" episode to the end of the series
Towards the end of the series, with this episode, Jack, thinking they were both dying, told Furley that he wasn't gay. After they were subsequently rescued, Furley relayed this to Larry, Janet, and Terri. He said that Jack was delirious, which made him say this. But, in the episodes following this one, the gay jokes were fewer, and the ones that were there were usually followed by a "just kidding." Also, during the final episode, before Jack tells Furley that he's moving in with a girl, Furley is obviously within earshot several times of Jack and Vicki and their situation. He's one of the ones listening to the conversation in the kitchen during Janet's wedding. He just didn't let on that he knew because he saw Jack as more of a friend/son from here on out.
 * And he couldn't just come out and admit he knew the truth, in an earlier episode he said if his brother(who actually owns the building) found out Jack wasn't gay then he would make him kick Jack out.