I grew up in a heteronormative world thinking I was a heterosexual male. Tons of crushes on girls. When I was in high school I started having crushes on a few guys. By the time I became an adult I knew could be emotionally attracted to men as well as women though way more women than men.
I ignored or turned down several guys in college who flirted or asked me out. Near the end of senior year I had sex with a guy who invited me to his room at the hotel where I worked.
No one guy or event "triggered" it. I gradually became aware of it.