I know this has been a topic many... many times before, but is it not possible that masturbation may have an effect on gyne.
Categorically- NO.
Masturbation is chemically exactly the same as sexual intercourse. Sexual intercourse and sexual behavior are generally at their highest between late teens and early thirties. This is exactly the time when statistically development rates of gynecomastia are at there lowest.
As well as puberty, gynecomastia also readily occurs during two other points in life. It occurs in babies and it occurs in those over the age of 50. These times of course do not fall in line with this pet theory.
What you are referring to would equate to a negative feedback system that would result in the most proactively sexual people being statistically far more likely to develop gynecomastia. It would also mean within such groups a statistical likelihood of the most active developing more significantly severe gynecomastia;
But neither of the above has any semblance of truth, neither reflects anything approaching the reality.
In fact quite the reverse gynecomastia is at its root caused by a hormonal imbalance be it temporary as in puberty or more permanent as in hypogonadism or liver disease. Many of the conditions that cause gynecomastia actually cause lowered libido and with lowered libido you have reduced sexual activity.
Your negative feedback theory would in effect result in a greater estrogen to androgen ratio which would in turn, turn off the very feeling that leads to sexual activity.
So you are proposing a mechanism of gynecomastia development that would actually be opposed by the physiology of the body according to how the body would work if such a mechanism existed..
I can tell you from an endocrine point of view that this is an absolute non runner- and I am being very generous in putting it that way.
No creditable endocrine source anywhere in the world buys into this theory.
No harm in asking though...
For the real causes of gynecomastia look here;
http://www.gynecomastia.org/smf/index.php/topic,8499.0.html