On that point, being bi I've actually experienced this from one lesbian partner. It wasn't something I did, it was their own anxiety and insecurities which were triggered by anything that could be a potential abandonment risk. She could rationally understand the fact that me being able to be attracted to any gender doesn't mean I am currently actively attracted to others or that I'd never choose the "easier" gender to be with. But when her anxiety hit all she could think of was how she'll be left behind for a man. It was tough on both of us and eventually broke up, but she started therapy when older and it tremendously helped her!
So even though this is a fictional work, let's be more apprehensive when criticising people who show this kind of behaviour in real life and try to understand them.. Cause that's how they'll have more opportunities to find ways to overcome whatever is challenging them and holding them back from healthier relationships
Everyone dog piling and shitting on the MC for how he's feeling clearly does not know firsthand how complicated such feelings towards someone you've known for a long time can feel like. Especially when you've known them to be a certain way up until that point, it's like your whole world gets turned upside down so it's natural the MC was going through some withdrawals and insecurities.
Nothing to shit on him for because he never blamed the ML for who he was or shamed him for his sexuality which he obviously can't change. I always notice this trend of BL readers always liking to shit on the more "feminine" guy in the relationship.. disgusting