I've never felt my sexuality to be fluid. I still remember the summer when I was 12 and girls suddenly became interesting and scary in a way that required absolutely no thought or reflection on my part.
For most folks it doesn't feel fluid at all - it depends on numerous factors with environment being the major factor. The entire issue is quite complicated and cannot be easily summarized, although IMO the following article does an excellent job of it: How might homosexuality develop? Putting the pieces together
What!!? It took you 12 years to know it?
I've never felt my sexuality to be fluid.
I still remember (when I was about 12) the first time my sexuality became fluid .
Mine is. I drift from blondes to brunettes to redheads, all in about an hour.