Sad sex is when you feel you are losing all touch with her, and she with you, but this is a way of telling one another that the connection is still there, somehow; that neither of you is giving up on the other, even if part of you fears that you should. Then you discover that insisting on the connection is the same as prolonging the pain.
— The Only Story (Julian Barnes), Loc. 1819–21
And:
Sad sex is when she is sober, you both desire one another, you know that you will always love her regardless, just as she will always love you regardless, but you – both of you, perhaps – now realize that loving one another does not necessarily lead to happiness. And so your lovemaking has become less a search for consolation than a hopeless attempt to deny your mutual unhappiness.
Good sex is better than bad sex. Bad sex is better than no sex, except when no sex is better than bad sex. Self-sex is better than no sex, except when no sex is better than self-sex. Sad sex is always far worse than good sex, bad sex, self-sex and no sex. Sad sex is the saddest sex of all.
— The Only Story (Julian Barnes), Loc. 1828–33