Even though I'm all for everybody defining sex for themselves, this is sort of an emotional topic for me. I guess I'm fine with any definition as long as it comes from questioning your internalised beliefs and seeing how you really feel about it. What I'm not fine with is that often anything besides man-woman intercourse is labeled non-sex and less than.
Kissing alone isn't sex for me, anything more than that is already in the grey area of sexual action. Hand jobs are most definitely definitely sex for me, and so is mutual masturbation. Masturbation is sex, too, just not with another person. And BDSM stuff goes either into sexual action or sex category, depending on what it's like and how it feels (and yes, the lines are kind of blurry and, in the end, not very relevant).