It's not necessary to believe in a fictional world to be a fan of it, or even think it's all that likely.
But the Federation has lofty ideals, just as America does, but it doesn't mean it, or we, are perfect or always live up to our ideals. That's realistic, too, and often portrayed quite well in Trek. So they suggest they have eliminated some bad elements completely, but I'd just assume they are often overstating the case or talking about an ideal, but it's far better than before, even if still not perfect, and with minimal needs met in society, one isn't forced to do harm just to live. But live well? Live better? That still takes work. To live with purpose? That, too, takes effort. But the goal isn't to simply feed and clothe yourself so much as to improve yourself. Not to become better than others, but simply better than you are now.
Even there, I do not think most members of humanity would wish to become members of Starfleet. Their ideals are perhaps higher, harder to conform to, and would tend to require you to work hard, leave many things behind, like your prejudices, or even belief systems that insisted other beliefs must be wrong. They are a more idealistic bunch dedicated to tolerance and pluralism and equality. But striving for it never means they have achieved it or that they are perfect. I believe in that.