Even when it is happening right in front of you? I mean not to knock on anyone's kink of course!
Well I'm not really monogamous to begin with, the only reason I'm in monogamous relationships is that they work better for me for practical reasons. But I don't intend to possess people and I don't mind my partners having sex with other people, even in front of me, as long as my consent was taken into account, as with any other pda or sexual thing. (depending on the situation, that might very well be pretty nice ;) ) So to me, cheating is just breaking one of the rules that were set in the relationship - it's bad, sure, it can even be pretty terrible if you know it will hurt the other person badly, but the act in itself is not the end of the world. It's your body and your freedom to begin with.
Never cheated on anyone and never got cheated on tho. Who knows how I might react !
I would not like to be in Nanase situation tho. But mostly because I don't like to see sexual acts or sexual pda from up close without consenting to it first. The cheating is irrelevant to me.