not that defining sex/notsex is even useful if you're not religious/desperate to lose virginity, but i'd tend to think of it (out of habit/convention than any particular reason) as requiring penetration by a real or fake penis, or a serious amount of mutual masturbation/oral where penises are absent, with everything else as "sexual activity".

speaking of continua, i was thinking recently that the gay-straight axis should be a triangle, with asexual on the other point. but then "sexuality"'s even less quantifiable than orientation.