Not the most fun conversation topic, but sometimes I wonder if sexuality can be shaped or influenced a lot by previous experience with sexual abuse or very early exposure to sex.
I highlighted those words as I beleive they are quite important.
There is a difference between being biologically aligned to a sexuality (ie, born gay or straight), and what you are suggesting with your post.
Normally, as we grow and our brains start to mature, we embark on our sexual journey on our terms. By that, I mean when
you are ready, you engage in sexual behaviour. Generally, when we are younger, we play around with both sexes to an extent. It's all about figuring things out, and navigating our personal maze to find out answers that make sense to us. So, while I may have played under a blanket with my female neighbour when I was younger, I was still gay but didn't fully accept/realize it until later in my teens. It was experimentation
on my terms, that I processed
on my own time, in a chain of events that led to me naturally becoming who I am today.
Premature exposure to experiences negates the natural process of figuring things out on your own, when you are ready. It happens on someone else's terms, and shocks your system. You simply aren't ready to deal with it, and it forces your brain to deal with something it simply isn't mature enough to properly process.
This can do a few things to a young mind:
1. Completely block the event. The brain just says NO and buries the experience. These tend to resurface later and cause a lot of problems that only a skilled professional can untangle.
2. Prematurely sexualize the person. One of the signs of an abused child is them behaving sexually much earlier than they should be.
3. Confusion and distrust, as well as acting out. There is an emotional/psychological soup created when something like this happens to a young person before they are ready. Essentially, they don't know/aren't equipped to understand what happened, but their brain still tries to make sense of it, and they then attempt to mesh it into their own understanding of themselves and the world.
What it amounts to is
damage. You can also say shape or influence, but damage is much more accurate. Early exposure to experiences like that, no matter what their biological sexuality is, really muddies the water. Their natural growing and understanding of themselves gets overridden, interrupted, then confused.
That said, a straight child doesn't become gay, nor a gay child become straight, if this happens to them. I think that is the gist of what you were saying. No matter what their sexuality is, there will be a psychological deformation of how they see themselves, and how they behave.
(my caveat here is that this is a very short form, with a lot missing, of the subject. I was trying to answer the question, but realize there are many holes in the content I provided)