Hm, I've rarely had any trouble drawing the "what is sex" line, even though it doesn't particularly mean a whole lot to me. If anyone's pants come off I might as well just call it sex already, because something's going to end up inside something.
(I did have one experience that was difficult to classify - I was with two other guys, and I made out/fooled around with them both, and they unambiguously had sex with each other, but my participation in that portion was limited to things I consider not-sex, so I'm not certain if I should consider myself to have had sex with them. If that had been a candidate for my "first time" I might have been somewhat confused.)