“Oh yeah, guess I'm not straight”
I had a girlfriend when I was in highschool. But it didn't occur to me that I was therefore not straight until I told my friend and she asked me if that meant I was lesbian or bi.
Probably related, I met her online, where my persona was gender neutral with a preference for male pronouns (since they/them as a pseudocanonical nonbinary pronoun didn't exist then). My gf also went back and forth between genders as well.
I never really did the gender calculations in my head either. It's just one of those things I'm pretty blind to.
When I told my parents about her, I was also more terrified about them knowing I had a romantic relationship than that I wasn't straight. (Though of course they dismissed it as a phase when I met my current male partner)