Men are like a light switch in sex. In good health, when they think about sex or having a good sex partner, they get good sex mood and immediate erection. They ready for immediate sex act.
But in the case of women, it is not so easy. It is a complex process. Many more forces come into action like hormonal conditions, the quality of relationship, lifestyle and physical changes – menstrual cycle, pregnancy, menopause etc.
To make it simple, mental and physical arousal are two different things. Both men and women can be genitally aroused without any mental interest. It is a separate mechanism, which allows lubrication in women or blood flow to the penis in men.”