To be honest, I tend to think of my female students more as my daughters than as potential romantic interests. It's one of the reasons that I'm known as the school dresscode-Nazi; I wouldn't want my own daughter to wear some of the stuff that these kids show up to school in!
The only possibility that I can see is that the teachers doing so are as immature as the students. Unfortunately, I have met a few teachers who actually teach in high school because they loved high school so much when they were that age, and they want to relive it again. They get way too involved in the students' world. I can see how those folks might do something so stupid, but I just can't see how a rational adult could...