I'm pretty sure that its strongy implied that Lenny and Carl are gay. There was the episode I believe is when Barney gives up drinking, and Moe says something to the affect of "alright, this bar's going queer". Then in a few episodes later (maybe different seasons) its either Marge or Homer who goes into Moe's and sees Lenny and Carl their. Moe again says something to the affect of "They never leave".
That could just be over analyzedm its just something that pops to mind.
As far as Smithers being gay, its vaguely implied in a few episodes. When they have the gay pride parade, Smithers and one of Marge's sisters (I can never tell which one is which, probably the one who didn't marry Troy McLure) is shown on a "Coming Out" float. There singing something. He's got the worlds largest Malibu Stacy collection, Mr Burns startup screen, and a love for theater/broadway. And there was also the episode when Smithers learns of his fathers death. Mr Burns told him that a mob of women killed him when Smithers was a child, and asks him something like "I hope that didn't ruin you" and you see Smithers standing their pretty uncomfortable.