It seems this author is mentally ill as well as retarded. You can see it reflected on the characters of this and his other infamous work "My truth and your lies". They have no self-respect, no self-esteem, they accept any stupid demand they are made and do not have a single working brain cell.
In some way it reminds of another crappy manga (Guilty Circle), in which all characters have negative IQ, and yes, its author seems to have some mental issues as well.
Anyway, just read the description and the name of this manga. To sum it up: There is a girlfriend who "loves" her boyfriend and suggests swapping sex partners. (This is the first stupid thing to "justify" the unnecesary drama).
On the other hand, he still wants to be her boyfriend, despite what she proposes (which, of course, happens). (The ultimate demonstration of no self-respect, no self-love).
Basically, she is a braindead thot who says "I love you, but I won't have sex with you. I will have sex with someone else. You can either have your way with another girl or just watch me doing it with others."
To make things worse, she is not only the who suggests this sex partner swapping thing but also she lets this random guy do her raw (without using any kind of protection). It wouldn't surprise me if she ends up with some STD or having an unwanted pregnancy (and yes, unwanted and not unintended. If it were unintended, she would have asked the guy to use protection).
All in all, this is a crappy manga, with crappy characters, made only for the NTStupidity and NTRetardation lovers to like it, so don't expect any kind of character growth (or anything good from the author).