Many of the people that think being gay is a choice are in fact bisexual. So for them it's a choice on whether or not to act on it. They assume everyone else works the same way.
From a logical perspective that implication is true, choosing your sexuality implies you have a choice. However, I disagree with the premise that there is a choice to begin with.
I do not choose to be straight, I just am. I'm not gay, and no amount of choosing will change that. I'm a guy, and I can choose to look for a boyfriend, but it won't change the fact that I'm attracted to women.
Now maybe I discover that I'm actually attracted to both men and women, but I would argue that discovering is different from choosing. Choosing would mean that I can choose to not be attracted to a gender, which I can't. I can only choose whether or not I act on it.
Obviously they're bisexual, monosexuals are a myth.
You're bi or you're ace, and everyone is just one "being trapped on a deserted island with a hottie not of their usual preference" from recognizing it.
That's right, monosexuals, now YOU'RE being erased, deal with it.
As a bi person, it definitely was. It's a common joke bi people tell each other to make fun of bi erasure, that never lands outside of bisexual circles and yet we keep trying.