You're acting like everyone is just supposed to automatically accept that, very unrealistic expectations if that's what you're hoping for
No, I don't expect every love interest to automatically accept not being in an exclusive relationship with MC. However, I do expect MC to try to convince most if not all of his love interests to accept being in an open/causal relationship with him later in the story.
If the series is going harem ending, I can see all the love interests begrudgingly accepting MC's proposition for the time being. MC did tell Fuuka that he
likes all the members in his group before, and I don't remember seeing MC express any interest being in an exclusive relationship with anyone or having romantic feelings at all. If he gets confronted about who to pick later in the story by all of his love interests, I can see MC saying: "
I can't pick/ I pick all of you guys because I like you all equally and/or I love being an adventurer more.".
How the girls will react to this answer will depends on MC's charisma...
He wanted the harem, he's going to have to deal with all the different personality types he brought in. He's delusional if he thinks they would magically be okay with it without prior discussion. The setting is not a society where polygamy is normal.
I don't disagree. When it comes to sex with the members in his group, I think MC sees himself as a sex teacher/opportunist. All the girls are virgins with most of them have little confidence or knowledge in sex. If one of the girls is open to do sexual activity with MC to learn about sex or to simply blow off steam because they're so pent up, MC is not going to turn them down (with Io's case, he had no choice if he wanted information from the perv informant.)
But with that mindsight, MC obviously didn't think about the long-term consequences of his actions so now it's up to him to clarify what type of relationship that he wants to have with his love interests and for them to fall in line with his decision.