RAZD writes:We can rule out genetics.
We can rule out upbringing.
We can rule out "choice".
What's left is fetal development - the process that builds a fetus from the genetic recipe - and a mutation in the progress of that development in one fetus.
The analysis seems too simplistic to me.
In a computer, there's a circuit called a flipflop. It contains two logic gates (probably more, but let's keep it simple). They are wired up so that they have to be in opposite states. If one is a "true", the other will be a "false".
So maybe these are "flip-flop" twins. Perhaps they are innately ornery, competitive, and bound to be opposites. But it is arbitrary as to which is one way and which is the other.
I am not suggesting that's the explanation in this case. I am only saying that this is a possibility that you left out in your analysis. I expect there are many other possibilities also left out of your analysis.
My own view: We are not born homosexual or heterosexual. We are born sexual. And we have to find our own way to express that sexuality. There are probably many factors that will affect how we find our own way, including genetic factors, pre-natal environmental factors, post-natal environmental factors, plain chance, etc.
I suggest we just accept that everybody is different in many ways, and that we stop the discrimination.
Jesus was a liberal hippie