(Apologies if the language below comes off as cis-normative.)
I figured out I was interested in men at some point shortly after I hit puberty. I read a lot of fanfic (and wrote a little), quite a bit of it explicit, and the content with male genitalia was far more interesting than the content with female genitalia. At some later point I expanded my reading/writing horizons, and convinced myself for a while that I was bi/pan. That turned out to be incorrect, as I learned in college via more direct, live-action experimentation.
Skip to the better part of a decade later and I'm pretty sure I'm heterosexual, demi-romantic, and generally indiscriminatory about the gender when the action is above-the-waist.
So, no clean answer there. Sorry!
Because I'm shamelessly looking for new options to add to my collection: favorite "complicated" board game?