True enough, but it needs some further clarification.
First, let's establish the participants:
P1 and P2 are the prime interested parties.
P3, P4… Pn are the outside parties.
Px can be any party belonging to Cheating
Staring conditions:
P1 and P2 are in a relationship
Px is not in a relationship with P1.
Px is not in an (official) relationship with P2.
Viewer is assigned to either of the participants.
Process/Effects:
P2 is having sex with Px. P1 __does not need to be aware.
P2 is then a "Cuckold".
>"Cucking"
Starting conditions:
P1 and P2 are in a relationship.
Px is not in a relationship with P1.
Px might be in an official relationship with P2.
Viewer is assigned to P1
Process/Effects:
P2 is having sex with Px, with P1 being aware of it. P1 is NOT participating.
P1 is Deriving pleasure from that fact.
P2 is then an active and willing "Cuckold", the most despised kind.
>"NTR"
Starting conditions:
P1 and P2 are in a relationship, OR P1 has active interest in P2
Px is not in a relationship with P1.
Px might or might not be in an official relationship with P2.
Viewer is assigned to either of the participants, but most commonly P1 or Px.
Process/Effects:
P2 is having sex with Px, with P1 being aware of it. P1 is NOT participating.
P1 does not have to (and, by some interpretations, simply does not) derive pleasure from that fact.
>Orgy
Staring conditions:
The relationships between any of the parties are irrelevant.
The minimal amount of parties is arbitrary, but larger than 2. It is generally accepted that at least 5 or more parties need to be involved.
Process/Effects:
All of the involved parties have sex at the same time and in the same place, but don't need to have sex with every other participant.
>Threesome/Foursome/n-some
Starting conditions:
The relationships between any of the parties are irrelevant.
Process/Effects:
All involved parties have sex with each other at the same time. An n-some is a special subset of an orgy where, if we assume that sex is a transitive, symmetrical relation, then every involved party has sex with all the other parties. Direct sex between all parties is not necessary.
>"Swinging"
Starting conditions:
P1 and P2 are in a relationship.
P3 and P4 are in a relationship.
Process/Effects:
P1 and P3 have sex.
P2 and P4 have sex.
All parties are enjoying it. All parties might be seen as being "cucked", but they do benefit from it, so it's not the worst kind of cucking.
>Harem
P1 is in a relationship with P2, P3 … Pn.
P2, P3 … Pn are not in any relationships with each other.
Viewer is generally assigned to P1.
Process/Effects:
P1 has sex with P2, P3 … Pn, not necessarily at the same time.
P2, P3 … Pn might have sex with each other for the entertainment of P1, but don't necessarily enjoy it themselves (other than the happiness from pleasing P1)
P1 is the fucking alpha.
If the viewer is assigned to any other participant than P1, he can be seen as the ultimate cuck.
>Polygonal Relationship
Starting conditions:
P1, P2 … Pn are all in relationships with one another.
Process/Effects:
All parties have sex with one another, not necessarily at the same time, and all the other parties are aware of it. Any of the parties can be seen as being both cucked and cucking. The magnitude of cuckery can be measured in many different ways.
Is there anything I've missed?