I know how these things work. I felt the same way about most of my crushes, that they were unique and the most beautiful girls ever, but perceptions and even preferences change over time. The fact is that you don't know whether there isn't an even better looking guy with an even better personality out there for you.
It wouldn't be the same it has to be Ryan Gosling its not just about his looks its the way he is, you wouldn't understand.
What would happen if you met a guy that looked a lot like him, or even better\more attractive?
What if I wanted to have gay sex with Ryan Gosling but I don't actually want the gay part, I could just fantasize that my girlfriend's him.
The reason you fantasize is because it won't happen, and you wouldn't let it happen, so then it's just nothing, it's meaningless.
Is it wrong in my head to imagine chopping someone's head off? No, because I'm never going to do it. Do I fantasize about Katy Perry's tits? Yes, but if she came along right now I wouldn't be interested, the fun of the thoughts is that they will never happen, and you don't necessarily want them to because it'd wreck the fantasy.