Tbh I think their first time is rape.
Chapter 1, page 25: the first scene they have
http://www.mangago.zone/read-manga/sm_gokko/mi/30ba27864169047076448c274a9d7eb5_chapter-1/25/
In this moment he revokes consent and the seme gags him. They do not have a safeword, so the Dom cannot ignore this no (or rather shouldn't). They have not been together so the Dom can't know his limits or his tells for when things get too much.
The seme ignores the ukes revoke of consent.
Like I'm not trying to say you can't like this yaoi, but saying this was consensual is a stretch. It's dub-con at the very best if you want to be nice.
My first consensual BDSM yaoi...better than I expected. Wish more were like this.