Yea, cheating gf, not my cup of tea, especially when my ex gf did exactly that 5 years ago, funny thing that the reasoning was to be "more experienced in bed", like for crying out loud, if you want to gain experience, do it with the one you love, be more assertive and get it going. Or just do this kind of bs when you're not in relationship or after you broke up with the partner, simple as that.
I mean I know It's just a manga, but still this theme boils my blood.