A sort of related question is: why do we care? Why does it matter to label some sexual activities as "having sex" and others as "not quite sex" or "foreplay"?
I have always assumed the answer to that is "because sex is so fraught with cultural meaning, people want to know what they've done."
I never used to feel a need to define sex, until I lived with a teenager and found out that they STILL thought that if you kept your clothes on, anything you did didn't count. I knew someone who got pregnant that way in high school, and more than one who was very, very afraid they'd gotten pregnant, not having known sperm would be involved. (true!)
Talking to M (the teenager) I began to understand that the youth really get the concept of STDs, but often do endruns around their fears by thinking that only happens if you have sex (as does pregnancy.) Ta Da. "Real sex" means acts which can result in pregnancy or stds, which should only be done with people who matter to you, at least as far as girls are concerned.
So I have no urge to define it for myself, but ongoing horror reading a fanfic where, say, they've given mutual hand and blow jobs, frotted naked to climax, rimmed, and THEN (after months of this) have a serious talk about whether they're ready to have sex.
I guess it's the rescuer part of me.