[Aceywacey]

Spoiler:

Sekai would only be blameless if she did not know that Makoto was in a relationship with someone else. The fact that she does know, and in fact was the one who hooked them up in the first place means that she is equally to blame with Makoto for cheating.

Makoto was the one who couldn't hold back any longer, and Sekai was the one who let him know she liked him from out of nowhere - why did she have to do that? - and then accepted his feelings.

If you were a girl and you found out your boyfriend was cheating on you with your best friend (who led him on), would you say that all the fault lies on your boyfriend, and none on your best friend? Would you stay best friends with this person?

Both are equally to blame.