I have two examples to help explain my point of view.
The protagonist never showed romantic interest in any girl, so his interest in his male friend was no surprise at all.
Boku no Girl: The MC liked a girl from the beginning, which made me expect it to be a manga where the girl and the protagonist would remain together even after the MC switched genders.
The same thing is happening in this manga. I'm following it because I hope the MC will remain interested in women regardless of his body.
And again, just to be clear, if that's not the intention of this manga, that's perfectly fine, I just won't be reading it anymore.
I would not be shocked if the entire gyaru-fication process had made Jun bi. We just haven't gotten enough data in the main timeline to prove otherwise.
I also find it amusing that this version of Jun pretty rapidly figured out what was happening and found it scary.
I personally don't think the Transformation has affected Jun's mind in any way.
It would just feel veeeeeery cheap if the Gyaru Goddess just changed his brain so he would become the perfect Gyaru. And if she did, he wouldn't be having so much trouble adapting right now.
So, whatever he's feeling now, was always a possibility.
It's just that many Bi people don't realize they are until after they transition. They can't see themselves with another male while they are male, but the moment they change into female, now that opens the door.