Lmao, I totally understand your concerns, but, like, come on dude? Most manga is bound to have super unrealistic anatomy especially in manga that leans towards more the pornographic side, gay or not. If you want accurate depictions of drawn gay porn, I think you might need to check out a different place.
http://www.mangago.zone/r/l_search/?name=Mentaiko this artist's anatomy is much more realistic, I believe a gay man writes them but I'm not completely sure. Some of the plots are very silly, but still enjoyable reads

It's impossible to shove a dick inside an anus that has never been stretched before and it certainly wouldn't feel good, not even once. The shock and the extreme pain of it would likely make the bottom pass out.
In chapter 2 (the scene with the old man groping them both), the anus is drawn wayyyy too high up. The anus is actually positioned in between the cheeks, not at the tailbone as it was drawn in this manga. Aka incredibly bad anatomy.
Chapter 2 (the hotel sex scene), 3 (outside sex scene) and 4 (hotel sex scene): it doesn't matter how many times someone has fingered themselves. It won't be loose just like that. The ring muscles in the anus retract after they've been stretched out and will become just as tight as it was before. You still need lube to penetrate someone anally.
Chapter 4: "Tranny", really? Those slurs aren't even accurately used. But first and foremost, they shouldn't be used at all.
I'm a bi guy. I know about the sex stuff from firsthand experience but also from basic common sense. All I want is properly drawn and portrayed gay porn. Is that so much to ask for??