Hi, guys, I've read many BL mangas, manhwas and novels in what the sex scenes are often not realistic and there's the infamous "Yaoi hole" and many GL series where there's more emphasis on penetrative sex: in short, they are more roleplay of hetero sex than actual gay and lesbian sex.
I'm under the impression that many fanservice scenes in BL and GL genres are more oriented to cater the "boy-on-boy-is-hot" and "girl-on-girl-is-hot" fantasies than to actual queer desire too.
As I'm a cisgender heterosexual woman with no queer friends I could ask information, if I'm not inopportune, I'd like to ask to all queer people who follow this site what do you think it could be done to make queer sex scenes actually realistic? What homosexual and lesbian sexual practices are actually the most preferred by queer men and women? What do you consider actual queer fanservice? How would you write real queer desire? What series do you think depict realistic queer desire and sex scenes?

Not queer myself but since nobody else answered you...

I'm not particular knowledgeable on the subject but BL as a genre generally implies "mlm romance written by and primarily for women" and AFAIK its foundational works back in the day were not really written with any reference to or understanding of real then-contemporary queer people. Mlm works that were actually by and for queer men are generally categorized as a separate genre, "gei komi", literally "gay comics". (often called "bara" in the west but that term seems to be a slur in Japan so be careful with use) I haven't read any so I dunno whether gei komi would be more "realistic" than BL per se, but it would practically by definition be written with more "authentic" queer male desire.

I do know a lot more about Girl's Love/Yuri. Actual published yuri manga series are overwhelmingly written by women, who are usually also a majority of its readers (or reasonably close to it; some surveys report men as the majority but not hugely so). The general understanding seems to be that men who just want to have girls make out/have sex typically aren't willing to read through an actual story to get to that. As for the realism of "penetrative sex," AFAIK most real life sex between queer women does involve some kind of stimulation of the genitals by another object - whether a body part or external tool. If you mean strictly penetration by a toy resembling a phallus, my experience is that's relatively rare in yuri - or at least rare in actually-published Japanese yuri. I'm not nearly as into doujins, manhwas, or webtoons, so perhaps things are different in those.