I ordered that one yesterday. They're just fun. "I taught your girlfriend that thing you like" is an inside joke, so it has more meaning that "I'm a sleazeball" and if someone gets offended by the shirt then they're not really very open minded and/or could ask about it first if it really bothered them.
It more just makes me look like I could be a slut, but I don't care because I know I'm not. Meh.
Matt