This is a really old joke. I’m not sure when it started. It reminds me of the joke for fortune cookies where you just add the words “in bed” to the end of whatever it says. It can be really amusing how it changes the meaning. A simple yet effective attempt at humor, although slightly sophmoric. Biut then,who cares, as long as you get a good laugh out of it right? As far as the “that’s what she/he said” joke, my boyfriend in college’s roommate used to do it to me all the time. Almost anything you say can become a sexual double entendre with the addition of that phrase! It became ridiculous to the point where I could not say anything that could not be turned into a sexual comment. As you can imagine it got a little annoying at times. I think he wanted to embarrass me or something. That’s why he got a kick out of doing it.