Why does it only get classified as cuckoldry when it's a threesome with another guy? While a threesome with another girl is not?
Because the second case is 'cuckqueanery'. Same thing, female version of the world.
Which is pretty a pretty curious case for a non gender specific language like english. Maybe it comes somehow from french. In italian it would be 'cornuto' or 'cornuta' for the genderized word. Which, again, curious case, could be literaly translated as 'horny' with no correlation with the english meaning of the term.
And still, the threesome is borderline territory. Some would say 'eh, they are having fun', some other would say 'ok, but you still let a guy fuck your woman'.
At some point it goes sophism. Do we really care this much?