SUCH AS WE hate admitting that there is any aspect of the 20th-century sexual revolution we don't understand, we cannot say we fully grasp just what is going on in the panda cage at the National Zoo. The only fact of which we feel 100 percent confident is that if it weren't for the merest accident of their birth as something other than hominoids, Ling-Ling and Hsing-Hsing would long since have taken their rightful place on the "Donahue" show.

Just consider their history, replete as it is with all the latest and most faddish forms of would-be proxy parenthood, baroque extramarital sex, mate-swapping and spouse abuse. Is this not Phil's prime material? Can't you hear the calls coming in? Do you suppose Ling-Ling is going to take a cue from her human role models and try to charge that London "donor" a fee for bearing his panda offspring--that is, if it is his offspring and if she does produce it? And will good old Hsing-Hsing just sit by, as usual?

In truth, there is surely enough to their story to justify more than an appearance on Phil Donahue's call- in show. The TV program "Couples" would be an equally suitable setting. In our mind's eye we can see it now: that solicitous psychologist leaning forward gravely in his chair and putting the dread perception to Hsing-Hsing: "You say she always starts beating you up at the critical moment, Mr. Hsing-Hsing--but you really want her to beat you up, don't you?"

Gosh . . . it all seemed so uncomplicated Saturday morning when the news of their amorous encounter came at last. Just like "Loveboat," we thought. Just like all those paperback novels with pictures of southern belles on the cover. Just like Paris and Helen and Tristan and Isolde and Anthony and Cleopatra and Oliver and Jenny . . .

But we run on. And, anyhow, given all the turbulent, modish adventures they've already had, it was no doubt too much to hope that Ling-Ling and Hsing-Hsing would, at this late date, indulge the simple, romantic dream so many of us had for them. Whatever is going on over there at the zoo, once again it sounds anything but simple or romantic. This is the age of the matter-of-fact, the clinical, the unsentimental and the unashamed. And they don't let you forget it for a minute. Not even at the zoo.