I used to think just like you. That is, until I experienced fluidity myself. Arousal, desire, enjoyment, passions very well can waver for a lot of people. And it can also take time for someone to truly understand the dimensions of their “sexuality” and of the gender, emotion, romantic, relationship contentment aspects of themselves. Because we’re so obsessed with sexual behaviors and identities, and because sex is so connected to ego for most men, it can take a very long time for men to completely understand themselves. Some never get to that place of self-understanding or self-comfort. That’s even for many “out” guys. They think embracing an identity or having sex with guys is the be-all of their journey then get frustrated later. But yes, “fluidity” is a thing and a potential aspect of many people’s “sexuality”. It’s not just some theory many people decided to collectively make up.
However, yes, a lot of these men claiming to be “gay4pay” or “curious” or “open-minded” or claimed to experience fluidity are lying. Many have and have always had fairly stable dimensions. They just weren’t willing to accept it. Many have a lot of insecurities and mental health issues and constantly deal with internalized phobias and misandry. Many are ignorant when it comes to dimensions and the spectrum. The ultimate problem is that there’s still too much weight put on sex and identities. While there’s still a lot of social equity attached to “straightness” and masculinity, even among “queers”. That’s why pushing “gay4pay” is still a thing. That’s why masculine/“straight passing” privilege and worship is still a thing. That’s why “queers” trying hard to stay attached to hetero-normalcy and hetero appeal is still a thing.