Some combinations, which may render a person’s impotence, are:

Rahu or Saturn in second, Mercury in 8th, and Moon in 12th house.
If Moon is hemmed between malefic and eighth house is occupied by Ketu or Mercury.
If Saturn and Mercury are in the eighth house and Moon is afflicted by Rahu, these are a few of the combinations for rendering a person impotent, but it is not permanent unless physically damaged.

Planetary Effects On The Nature Of The Organs by
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If the eighth house belongs to Jupiter, the sexual organs will be of normal size and function.
If the eighth house belongs to Saturn, the organs will be longer and languid with abnormal functions.
If the eighth house belongs to Venus, the native has beautiful organs and they enjoy their intimate life.
If the eighth house belongs to Mars, the organ will be small but the desire will be high and the person will be sensuous.
If the eighth house belongs to Mercury, the native will suffer from an inferiority complex in regard to sexual matters.
If the eighth house belongs to Sun, the person will have virile organs and the functions will be normal.
If the eighth house belongs to the Moon, the person will have normal organs and functionality will depend upon the mood of the person.

The planets occupying the eighth house change the above results.

Mercury, in eighth, not in own house and not aspected by benefits will make a person disappointing in sex life; otherwise, it will be a normal function. Rahu in eighth makes a person maniac for the act and Ketu imparts hypersensitivity for the same.

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Author's Bio: 

Sri Tulasi is a professional astrologer, life coach, and member of the Indian Astrologers Council. She has more than 15 years of practical experience in astrology. She is a regular columnist in many leading newspapers in India.
Visit her Website Spiritualsadhana