I was outside in my yard the other day and I noticed my new neighbor was outside as well, so I hollered over the fence to introduce myself.  I said to the fella, "So what do you do, buddy?"  And he says to me, "Well, I work down at the university over there.  I'm a professor on logic."  So I says to him, "Professor of Logic, huh?  What's that all about?"  And he says, "Well let me give you an example.  I see you got yourself a doghouse there, so I can guess you've got a dog, is that right?"  I said, "Yep that's right".  He continues, "And if you got yourself a dog you probably got yourself some kids, is that right too?"  I said, "It sure is."  And he continues, "So if you've got kids, you probably have a wife, is that right as well?"  I said, "Wow, yep, that's right too!"  And he continues, "And if you've got yourself a wife, then you're a straight male, right?"  I said, "I sure am".  So he explains, "So you see, simply because I saw that you had a dog house I can deduce your sexuality."  I said, "Wow!  That's really interesting!"
Well later that day I'm down at the bus stop waiting for a bus and I strike up a conversation with another fella that's waiting too.  I said, "I met my new neighbor today.  He's a professor of logic down at the university, there.  Isn't that interesting?"  And this fella I just met says to me, "A professor of logic?  What's that all about?"  So I says to this fella, "Well it's easier if I give you an example.  Do you have a dog?"  And he says, "Nope".  So I says to him, "Oh, so you must be one of those gays!"