Looks like I have to repeat the same thing over and over again.
"Event/sample space
Event is what you want, whereas sample space is all the events related to that.
To see if some factor has a lot of impact or not in some behavior, we check that %. We can get all the % with many events related to that, and we'll have arround 100% when we put them all together(arround because of the lack of some information, or the lack of some numbers because of the research).
With this, you would see how much impact does "being raised by gay parents" account for. And with the same %, getting the way we calculate probability, we would use that % as well.
If the other factors are just as important, then we would get closer % of them, as they would have more impact, and therefore, more numbers.
Why do you think that the % of kids raised by straigth parents is lower? Because of how the parents act, and what they teach the kids. The numbers show how much impact these factors have.
Now think: why is it increasing? Because now, kids see it in the TV as well, because now, parents tell them it's ok. And now, some parents act that way as well.
It's called being logical. If that's a problem or not, it's up for society to decide, as I said."
Try reading this.
As I said, the probability is not a "for sure" thing. Just becaus the numebrs are higher, it doesn't mean it will happen everytime.
However, we can see the impact of the factors based on those numbers. The factors, you mentioned, the bold part, obviously has it's weight. However, that weight can be seen if you get the numbers.
Of course it wouldn't the 100% accurate, as the only way to achieve this, would be making the experiece you mentioned. The "segregation". But as you said, that it's impossible.
However, we call still account for some accurance with the above. And let's not foget about the psicollogic works as well. Many psicologists are sure that at younger ages, kids copy many things they see. As their closest exemple is their parents, they would repeat their actions a lot. And this would have an impact on the kid's behavior for all his life.
It will also depend on what the parents teach them, as some parents do some stuff (throwing trash in the street, for exemple) but still teach the kids that it is wrong.
All of this, plus the numbers, can't be ignored.