First off, no this is not another post about staying away from your students. It IS a post about a new study that has implications for your interactions with your students.
The journal of Sexual Health, as reported by Salon.com, surveyed adults to determine how they define sex. It turns out there is little concensus on what acts are included when you use the term "sex." It's fascinating and a little baffling, especially the 95% number (you'll have to read the article yourself to find out what 95% percent say - and what 5% DON'T).
As the Salon article points out, the results are important because they have implications for how we talk about sex with each other and, in the case of educators, with students. Teenagers in particular are already very good at being as literal as possible when you give them rules or instructions, so if you have reason to talk about sexual behavior with students, or anyone else for that matter, it's vitally important to know that we don't all share the same definition for the word "sex."
And in the interest of clarity, I'll just say that intimate contact between a teacher and student is always inappropriate and usually illegal, no matter what word you use to describe it.