It legitimately is cheating. The plot is stupid just like all Japanese work, every single one. The very first chapter the chick believed a womanizer's words and just barebacked a dude who told her that her first boyfriend cheated. Skipped through the rest and it was just blatant cheating, could have just been a hentai for what it is but they decided to make a borderline Hentai manga of all the ways people cheat!? It will get you angry unless you have no emotions or never expect to have a lover.