"Washington is so incestuous," announced the ex-New Yorker to the newly arrived Bostonian.

"Look," he nodded his head toward a former famous couple, now each remarried to another. All four were sharing the same dinner table. "No one really splits up here, they just add on."

And on and on, until so many exes mark the spot that a host or hostess knows any large party will contain combinations of couples who have shared more than the cheese dip.

Guests at a recent Washington wedding included not only the groom's ex-wife and the woman he had lived with for four years, but three former girlfriends to whom he had professed varying degrees of passion. Each of the exes shed a sentimental tear, and not one of them cast an eye skyward when wishing the bride a happy life.

Very proper behavior, but this was a man who obviously knew his exes. Sadder is the tale of the dense young man, a do-gooder if labels must be given, who saw only good in others, even when it wasn't there. Which is how he came to arrive at a dinner party in the company of both his present girlfriend and his ex-wife. They had so much in common and he liked them both so much that he wanted them to get to know each other.

Midway through dinner, hissing sounds could be heard from their end of the table. The hisses turned to snarls and as the brandy was being passed around, the girlfriend rose to issue an ultimatum: "Come home with me now." The perplexed young man remained seated, whereupon the girlfriend screamed a few things one does not usually scream in public and left.

Good entertainment, everyone agreed, but not exactly the makings of a civilized dinner party. What one should do is be very pleasant if possible and, at the very least, very polite. This is easier to do when the encounter is not a surprise. In that case the two women, who arrived as consenting adults, could not deal with the situation. But generally, couples manage very well so long as the encounter is not a suprise.

No host or hostess should fail to warn someone that an ex will be present.

One Washington woman, knowing that apportioning friends is much more difficult than dividing the china, simply tells newly split couples, "I am both your friends. When I have large parties, I will invite you both. If you can't cope with that, work it out yourselves. One can come early and leave by 10. The other can promise not to arrive until after 10."

Another woman has worked out an even more sophisicated approach. Both she and a close friend have ex-husbands who live out of town. When either husband arrives to visit the children, a trade-off takes place. Husband A stays at the house of B, while Husband B uses A's spare room. Both ex-husbands occasionally oblige their hostesses by taking on the role of extra man at dinner parties. This has not yet led to any crossover romance between the A's and B's, but people versed in the complexities of Washington couples are watching the situation with interest.

One older ex showed up at a family dinner bringing his new girlfriend, who spent the evening playing cards with his children and their spouses while he and his ex-wife argued.

Both time-sharing and ex-sharing are based on the assumption that wrecked relationships are a social liability. But some hostesses have learned to use them to advantage.

The ex-fiance' and the current husband have both been drilled into proper behavior by the woman one had wooed and the other had won. When they meet, neither shows any signs of jealousy, but their sense of rivalry remains: They outdo each other in the telling of witty stories.

A hostess who knows what form their jealousy has taken invariably invites them both to what otherwise would be her dullest parties.

There are other ways to take social advantage of former relationships, as in the case of the Georgetown woman who has strewn the city with rejected lovers. People who have nothing else in common have either had an affair with her or had an affair with someone who has had an affair with her. For awhile, it was thought the only social event it was safe to invite her to was an evening of solitaire. But then some hostess discovered that her presence at a party was a plus, assuring a meeting ground for guests who might otherwise have had nothing else in common.