What does "non-binary" mean when a person uses the word
to describe lifestyle/sexuality choices?

I'm pretty sure that gender identity is not a lifestyle/sexuality choice. I know that I never arrived at a point in my life where I had to choose what gender I wanted to be and I don't think anyone does, it's an innate thing. I assume that someone who identifies as non binary feels no strong affinity to one gender or the other or else sees themselves as being flexible - good for them, they get the best of both worlds.