Studies have shown that over tens of thousands of years the ratio of handedness among humans was about 10:1. Although there was an odd dip over the last few centuries to about 3%. A recent study shows left handedness is again approaching 10%. A study has attributed the following factors in this dip.
Left-handers suffered severe prejudice during the 18th and 19th centuries and it was often "beaten out" of people
In adulthood, left-handers were often shunned by society, resulting in fewer marrying and reproducing
As prejudice declined in the 20th century, the number of natural left-handers who stayed left-handed increased
I will suggest it is possible a similar issue exists with regards to sexual orientation. Social pressures may cause bi and gay men and women to hide their sexuality and live a life that is contrary to how they are "wired."