Your take makes sense too, btw and I agree that Erina wants it both ways. Well, I don't know what was rolling around in the writer's mind so my conjecture is going to be creative at best and, unfortunately, I have no desire to read thru this story again so I hope I remember enough to respond knowledgeably. I don't remember Erina being aware of Ikkun being a sexual tyrannosaurus at the very beginning, but I do remember her thinking some disingenuous stuff about him while the three were hanging out. Stuff like, "He's too overconfident...". Not being cocky (lol), brash, rude, braggadocios, narcissistic but overconfident. Being "too overconfident" sounds like there's too much of a good thing, which in this case, I don't think exists. So I think this is another tale of the unfaithful and immoral chick trying her darnedest to convince herself to deny her overall attraction (not just he might provide a pornstar level lay) to some dude, only to succumb to her own temptation. But yeah, she probably realizes the overall quality of a relationship with somebody like Ikkun has a low ceiling so she wants to keep the sensible option off to the side.

For gammenon:
Erina wants it both ways.  She's hot for Ikkun's big, fat cock, but she has no interest
in him as a boyfriend, so she's unwilling to give up her current boyfriend because he
is her "official" social companion---the one who squires her around in public.

And as for Erina, that's ok - just end it with the boyfriend and be honest about it. She's in it for the dick, nuttin' wrong with that. I just really hope the boyfriend would understand that if she wants to be part of Ikkun's waiting list harem, let her [go] and find somebody else. It would be satisfying if one of these type of stories had a scenario where it blows up in everybody's face.