Not a statistics problem, but I'll see what I can do.
World population - 6,973,738,433.
Percentage of world population male - 50.25%
World male population - 3,504,216,824
It is estimated that 26% of the world population is under the age of 15, and 30% is under the age of 18. I'm going to estimate that 23% is under the age of 13, and consider that the average age where ejaculation begins.
It is estimated that 6% of the world population is over the age of 70, and I'll consider that the average age where ejaculation ends.
So 32% of the world is outside the age of ejaculation.
68% of 3,504,216,824 = 2,382,867,441 ejaculators.
It is estimated that the average male has a sexual outlet five times a week on average. That would be 261 times a year.
261 x 3,504,216,824 = 621,928,402,019 ejaculations per year.
The average amount of ejaculate per orgasm is 10cc is one teaspoon.
621,928,402,019 x 1 tsp = 621,928,402,019 tsp ejaculate per year.
There are 768 teaspoons in a gallon.
621,928,402,019 / 768 = 809,802,607 gallons of ejaculate per year.
Lex