cuckquean fetishes are usually women self-inserting as the cuck, just as cuckold fetishes are usually men self-inserting as the cuck. Sometimes it stems from voyeurism or compersion; but usually the cuck fetish is an offshoot of a submissive BDSM type interest.
It is much rarer for the cheating partner, or for the the other-woman/other-man to have very strong interest in cuckold/cuckquean fetishes. The cheater and other-(wo)man engage are just having, what is to them, vanilla sex; even if it is a kinky experience for the cheated on person.
ITT, mostly men into cuckqueaning are in it for the humor and schadenfreude. otherwise, they are usually interested in cuckqueaning because it is complementary to their harem fetish.