Ye, lube is essential for anal sex. I honestly don't understand why the author didn't make Ayumi-san use it... He seems like an expert regarding gay sex, and those magazines should have taught him at least the basics? So weird lol
For the story to make more sense, I always just assume in every story the seme uses lube. The author just chose not to show that scene. It fills in a lot of issues with plots.
It's a great manga and all, but dicks normally don't slide in with just a little bit of licking lol