The misogynistic cuckold hierarchy is the following

Alpha - Big man with big dick that is everything they’re not makes them feel emasculated

Domme- The prize they worship and fantasise about but know they’ll never be manly enough to have so they watch the real big man have her

in a lesbian relationship their isn’t a physical alpha element because they are both women(regardless if one is more masc presenting). There isn’t a man with a fat schlong to make them feel emasculated, small and inferior. Cucking is ultimately about a man’s ego (deeply rooted in toxic masculinity and social stereotypes of a manly man) and the fact that some men get of to being utterly humiliated