On School Days

I am only completed half of the show so far. End of episode 6.

I feel that Sekai is ruining relationship between Itou and Kotonoha. Not because she sleeps with him, but rather because of how she handled it in the early stages of the relationship.

She so much bugged Itou with her incessant advice on how to be a proper boyfriend, that caused Itou to get tired with relationships very fast. Thus, Itou wanted some easy gf, with whom he would not have to try hard. Ironically enough he found it in the very Sekai, who burdened him with advice on being proper bf.

In her own relationship with him, she says nothing critical of him and only feels guilt for stealing him from Kotonoha.

In Kotonoha's defense it was not her who wanted any of that. Sekai was burdening Itou with advice entirely on her own judgement. 

If Sekai would have left it as it is by the end of episode 1, Itou would have grouped Kotonoha like no tomorrow instead. Kotonoha has too little will to actually stop him on her own. However, with Sekai's help, they get Itou to 'respect boundaries'. That ultimately led towards him cheating on Kotonoha with Sekai, who is too hypocritical to use the same advice on her own relationship.

That said dakimakura has that advantage of never complaining no matter how you grope it, abuse, ignore or cheat on it.

In the same lieu, a docile demure girl, who will never complain or resist no matter what you do to her is the best type of girl. The most preferable type of girl.

 In the first few episodes, it was Kotonoha who had this quality and not Sekai.

Well, you can use gags, blindfolds, ropes and leather straps to make your girl as helpless and passive as daimakura, so BDSM is the way to go.