In the study of stellar evolution, there is this concept called the main sequence, a well-defined band that you see whenever you survey all the stars in the sky and plot their color on one axis and their brightness on the other. Not all stars fall into the main sequence, but the vast majority do; there’s even a lovely image of the graph here.
It seems the same is true of relationships. Stellar evolution and stellar nucleosynthesis map with remarkable fidelity onto relationships, I’ve observed, with a plot of “intensity of relationship” (as a function of emotional investment and expectation of continuity) vs. “sexual boundaries” showing patterns startlingly similar to the main sequence. At least to me.
So for example if you plot sexual boundaries horizontally and relationship intensity vertically, you might see something like this:
The sexual boundaries increase from left to right, with the classifications as:
A: Anything goes. Unbarriered, unprotected, full-on squishy fluid-bonded sex.
B: Barriers for anal and PIV sex
O: Unbarriered oral; no penetrative sex.
F: Fisting and/or fingering without barriers; barriers for anything else.
G: Gloves for fingering; no wet and squishy contact, even manual, without them.
P: Pants stay on; above-the-pants contact allows.
M: Makeout partners–no removing of clothing.
Now, not all the partners one can have fall in the main sequence. Along the top of the graph, we see partners distributed in Type Ia and Type Ib classifications: these are people you will schedule regular orgies with or a regular BDSM play relationship with, which may or may not involve sex (directly) but do involve a high level of emotional investment and commitment. Some of these folks might even be considered “family.”
If you’re part of the sex-positive community, you might go to orgies or play parties on a regular basis, and see the same folks over and over. These are folks you don’t necessarily have squishy sex with, but you might have some sort of irregular or semi-regular play/makeout relationship with. There’s not necessarily a high level of emotional investment, but you notice when you show up to a party and they aren’t there.
Type IV partners are most commonly found in poly relationships. These are the “Too Complicated To Explain” partners–they’re not necessarily partner partners, and they’re not necessarily part of the family, but they’re not not partners either…
A branch from the main sequence sometimes occurs for metamours, who a person might have some sort of sexual relationship with, but might not continue if that person’s partner breaks up with that person, but then again, sometimes these relationships do continue on their own, and…yeah, it’s complicated. Past a certain point, it’s not always clear from a single partner whether that person is main sequence or metamour.
A scattering of partners exist with a high level of sexual contact but a low level of relationship investment. These partners tend to scatter along the Friends with Benefits and One-Night Stand axes.