If you’re a teenager, this combination of familiarity and distance becomes a safe space for you to explore your sexuality and your preferences - and also just have fun! And if you write fanfic, it stresses on sexual fantasy being creative - instead of one dimensional. It helps you actually play with your fantasies, instead of talking about it sarcastically or jokingly, like we usually do.
It’s also been a safe space for queer and trans folks to find some representation of their desires and their identity - and also, representations of sex that aren’t just ‘penis-in-vagina’. It’s also fun to write the sexual tension you see on screen into existence — queer fanfic has birthed pairings that are so compelling that they might as well be canonical (the ‘real’ story) - because at this point, everyone knows Captain America and Bucky Barnes aren’t just ‘really good friends’.