I've suffered for days, I've sent it to my friends, I've told it on the mountain, all to no avail. Would someone please explain Cluin's joke of Nov.5-7:03pm?

Tarzan & Jane meet for the first time. They discuss everything imaginable and finally get around to sex. She asks what he does, seeing that he's been alone for so long. He points to a knothole in a tree & says I use that. Jane lays on the ground and says, as she spreads her legs, "come lay down beside me, I'll show you what you've been missing." As he lays down he makes a fist & punches her in the groin. She says, "Why did you do that?" He says,"I always check for bees first."