The vagina is kept naturally damp from its own fluids. This moisture is not sufficient for the act of love. The friction from penetration and thrusting demand that the vagina be really wet. Extra lubrication is provided from tiny blood vessels in the vagina walls. Early in sexual arousal, they become engorged with blood.
The pressure from the engorgement forces drops of moisture out of the veins and through the vagina walls. The drops run together and coat the vagina with fluid. The sexologists, Masters and Johnson, thought that this looked like beads of sweat running clown a hot forehead. Hence, the somewhat inelegant name: "the sweating phenomenon."
Extra lubrication is essential. Without it, the friction of a thrusting penis would soon become unbearable. Tiny lesions in the walls would occur, and the vagina would be at risk of secondary infection; other germs could get in. If the vagina feels dry, the sweating phenomenon has not occurred. If it has occurred, then some time has elapsed, and the woman no longer feels aroused. Stop all movement the moment the vagina feels dry. If wishing to continue, a break for more external loving should do the trick. A woman does not have an unerected penis to signal her lack of excitement. However, a dryish vagina is a clear sign to her partner that she is not sufficiently aroused.
The sweating phenomenon lasts into old age. At menopause, there may be less blood flow to the vagina, and so less fluid is passed through. A dryish vagina after menopause does not necessarily mean lack of desire. Just as a less-than-fully erect penis does not mean lack of excitement. These things just require a little more lime and attention. For the vagina, some external lubrication, will do the trick.