This is true for both men and women. The causes of impotence are more varied and complicated. Psychological factors play a major role for many men. Organic causes can include a variety of drugs and alcohol. Blood pressure medications and antidepressants are particularly bad offenders.
Impotence is also a consequence of diabetes or atherosclerosis.
This tends to be a circulation problem. See the pages on diabetes and cardiovascular disease to learn how to deal with these conditions. Gingko biloba has been shown to be quite effective in dealing with impotence brought on by circulation problems. Testosterone binding is a problem because it is free testosterone that powers libido. Testosterone can be increased by DHEA which has some libido increasing effects in some people.