One born under any of the twelve zodiac signs can have strong inclinations towards polygamy though some born under certain zodiac signs are more prone to such tendencies.
The twelve zodiac signs are broadly divided as moveable, fixed and dual and these are as follows:
Moveable
ARIES, CANCER, LIBRA AND CAPRICORN
Fixed
TAURUS, LEO, SCORPIO AND AQUARIUS
Dual
GEMINI, VIRGO, SAGITTARIUS AND PISCES.
One may have taken birth under any of the above zodiac signs and have a series of romantic relationships but once one gets married most remain loyal to their partners whereas some may indulge in clandestine liaisons or even have another secret marriage. This attribute is most common to those born under dual signs, which are also known as common signs. This polygamous trait is largely seen in those born under Gemini, Virgo, Sagittarius and Pisces, to a lesser degree in Aries, Cancer, Libra and Capricorn and rarely in Taurus, Leo, Scorpio and Aquarius.
However, one has also to consider the planets like the Mars and the Venus. The planet Mars is the driving force for sexual inclinations and the Venus holds the power of attraction and love. If these two planets are in mutual harmony in one's birth chart then a person may or may not have more than one relationship but they certainly remain loyal to the one they get married to. If the planets Mars and Venus are ill disposed in one's birth chart and are also under affliction due to other malefic influences from the Saturn or the Rahu, then one born under the dualistic sign will certainly have polygamous nature resulting in frequent liaisons and subsequent despair. Whereas those born under moveable or fixed zodiac signs under such circumstances may or may not have such tendencies depending upon other factors in their chart.
There is another very strong factor in a birth chart that can reveal one's sexual proclivities. At the time of birth the zodiac sign rising in the east is considered the Ascendant or the Rising sign, which is representative of the individual. The zodiac sign directly opposite to the rising sign, also known as the 7th house or the setting sign, is representative of the opposite sex. If in a birth chart there are the planets Mars and Venus in the setting sign then that individual will rarely remain loyal to any one partner whatever their zodiac sign at birth. Even the presence of the planets Saturn or Rahu, with or without the combination of Mars or Venus with them, will make one polygamous to the extent that could even be destructive to one's health.
In biogenetics we are gradually discovering that one's nature or characteristics are determined by the genetic code, through astrology the ancient seers had been able to ascertain one's mental and physical inclinations by calculating the planetary positions at the time of birth of an individual.
Hence whether one is monogamous or polygamous gets decided at birth.