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.
I'm open-minded but have zero attraction to the same gender. If we only need one counter argument against what you say to prove you wrong, there it is.
The "it's a choice" move is just to avoid having to discuss exactly why they believe gay people should not only be punished in this world, but for all eternity. Always move the conversation in the direction whenever they use it.