if we focus on this story and "accept" the premises, the important part is "how is it written".
what I mean is, what is the purpose of the size? to make the Mc feel inferior? to make Ayumi obsessed by its size like a cock-hungry slut and needs to be with Terrace without care of Mc's feelings or if that will destroy their relationship?
or is it just a enormous cock? the size will not create any issues between them. Ayumi is not obsessed, she is not seeing Terrace as the "better man" or that "only Terrace can make me feel good, not my Bfs tiny prick".
if I was the writer I wouldn't start with this. I don't mind a "huge cock" but its something for later. to introduce another dude I would use someone safe, someone shy, virgin, ugly, obese or really old. later when they have experience and all emotions are balanced, to add a bull could be fun for variety and experience.
still, nothing NTR has happened, so we can not complain. maybe it will be great, maybe the writer has something funny, dramatic or exciting in plans that makes the size sense.