You have a sexual arousal disorder when you continuously every time and again don't get a good physical reaction from sexual stimulation. This means that the woman's vagina doesn't get wet enough and that the man has problems with his erection at sexual stimulation. In order to get sexually aroused, something physical and something mental must happen to you. Changes of the body that usually happen when sexually aroused are a faster breathing, a faster heart beat, an increase in the saturation of the blood in the penis which makes it stiff, the woman's vagina getting wet
These problems often occur in combination with a reduced interest to make love
, sexual aversion
or orgasm problems
More. For men we talk about an erection disorder; for women we talk about an arousal disorder. These specific forms of arousal disorders are being discussed in other questions.