Is homosexuality an inevitability of evolution? Not in my opinion.
In my opinion it's a product of our uncivilized man made playground, and how it distracts us from what's important.
Here are two facts:
People can want to sex change because of the condition of their genitalia.
People will be gay when they experience imaginary disruption.
Walled-in, winding-road and small-boxed environments, disrupt imagination.
Not having access to clear sky and horizon, has a huge effect on your thought process. It's disadvantageous to a whole genetic type, it's a whole different train of thought.