first off, ten apple pie and ten pumpkin pie makes 20 pies, there's your fixed number, that also sound delicious. you're right, there is no law, besides math. math is the perfect law because you can't be punished for breaking the laws of math because they can't be broken. 1+2=3 and it can't be changed.
second, when someone makes the decision to be gay and is open about it, that's when red -1 and blue +1. duh. you learned addition and subtraction in kindergarden.
third, US census. they keep track of the population, people's sexual preferences, age ratios, where you live, the computer chip in the back of your brain, etc. there is a fixed number and it's the number of the population that the US census records each year, or whenever they record, i dunno. go google "gay/straight ratio" and you'll find that 1% of people are openly LGBT (now this is the percentage of people who are open about it, not including closet LGBT). this means that 99% of people are straight (omitting closets LGBT for now). now, if in the next 20 years, this percentage changes to 10%, that means 90% of people will be straight. either way, it adds up to 100%. so long as both percentages add up to 100%, it's a perfectly logical ratio.
this isn't meant to be bias or hateful in any way or form, it's math, and math is the most unbias thing there is, besides quakers, quakers are friggin awesome people at heart.