One of the most unrealistic things about anal sex in yaoi is the lack of lube in most stories.
Ikr! I always wonder how come uke are never lying in hospitals beds with their ass hurting terribly
Not necessarily. It depends on the person. I used to wonder about that myself, so I asked a gay friend and was informed that there is a lot of variance. Artistic license aside, the virgin uke usually isn't about to endure something as hardcore as fisting, double penetration or heavy slamming, so apart from the usual amount of foreplay and taking it slow to aim properly and enter with caution, it doesn't require much more relaxation or prep than ... well, a regular BM. For some, of course, it does take a lot of time, patience and gentleness, but not everyone.
That aside, mangakas usually are under editorial, spatial and time restraints in which to tell their story, so certain formalities do tend to be dispensed with for the sake of brevity. There is also the romantic trope that, if the readers aren't ready for the sex by the time the characters get to it, the prep won't help, and if they are ready, the prep is a delay. Romantic tropes don't suit everybody, but the editor usually decides what stays or goes during the design process.
In many Yaois, when the uke is having his first time, the seme fingers his ass to loosen it up and penetrates soon after. But, doesn't it hurt? Because Yaoi often gives us unrealistic expectations of how sex is supposed to be, wouldn't it be better for the uke to finger his ass days before his first penetration so it can be smoother and easier than the seme fingering his uke's ass and penetrating for the first time on the same day?