Probably the best/shortest explanation of this analogy that I have heard is "The Four Fs": "French", "Fondle", "Finger", "Fuck". This corresponds directly to each base in masukomi's writeup above:
- French: refers directly to French Kissing, but more generally to all sorts of kissing, hand holding, and the like
- Fondle: cuddling, spooning, feel up, among other things
- Finger: fingering, mutual masturbation, oral sex
- Fuck: is any explanation necessary?
Right, I think I just heard my purity test
score drop through the floor
(yes, this writeup is very uncharacteristic of me; I was shocked when I visited this node that there wasn't already a "four Fs" writeup here, which with the vast depth and breadth of sex-related content on e2, so what could I do but throw in my two cents?)