Since I was a kid, I failed so hard at being a guy. I've always been hopeless at athletics. My body type has always been pretty meek (let me not doxx myself and say more). I hate any sort of competitive environment. I can't hold my liquor for shit. I have a very high pitched voice and an expressive way of talking. Friends have described my voice as a "gay twang". My mum probably assumed that I was gay from day one, as I got a lot of "it's OK to be gay" from her growing up. Sadly I had too much soy or not enough soy, because I grew up attracted to women.
Maybe you old comrades remember, but schools in the 90s were full of homophobia. "That's so gay" criticised any action that deviated from some masculine ideal. I got this multiple times a day, and I learned to stifle my personality to avoid the rebuke of my male non-friends. I'm not even complaining, there's so so many that had it way worse than I did.
Nowadays it's great being a flamboyant straight dude. I can be as sweet, as empathetic, and as expressive as I want. I have cute and colourful clothing. I get really ecstatic around animals. I cry. People like me for being fun and engaged with stuff. Nowadays if some guy colleague says that's "gay" it's like 
"are you alright mate??"
I did go some LGBT events and actions in the past, but not a lot. If I do anything positive, it's to enforce no homophobic language with my students, which guys has gotten a lot easier in the past 20 years. Really, the kids nowadays are much better than we were. OK, I have hooked up with a few dudes here and there, but it feels like stolen valour to call myself bi.
So thanks a whole lot to all the queer people who have made my life much easier, when I've done so little.
Adding that, it's perfectly valid to say you're bi if you would never consider having a long term romantic relationship with a man, you only do that with women, but you are willing to have casual hookups with men.
Yeah this honestly sounds like a degree of internalized biphobia from growing up in a backwards era. I wonder if OP grew up today they'd have different revelations.
I convinced myself I was 100% straight despite same-sex attraction until about 14/15, solely because I could only see myself with a girl. Looking back it was cognitive dissonance, I very much wanted to fuck guys and that's pretty bi.
I also think I mind-blocked myself idea to the idea of same-sex romance from some pretty strong heteronormative societal conditioning. I feel like sexual attraction is uncontrollable, while romantic gender attraction is more mental and environmental, but with decades of conditioning, is probably extremely hard to change. There's plenty of gay men who'll tell you they are/were into guys exclusively sexually and women exclusively romantically. But afaik, sexual attraction is the main definer for sexuality
You can be bisexual but only hetero/homoromantic.