You both know the other one cheated on you so you cheat on them to get back at them and then they cheat on you in revenge.

Me too, maybe like an abusive relationship?

I had a mother in law who stayed with her abusive husband because he threatened to kill her and their children if she tried to leave. Seems pretty unethical to me.

A less extreme example would be one partner badgering or manipulating the other into being monogamous when they don't want to be.

Perhaps old school runescape?

Any non-dead mmo

I want a dark souls love story.

"When they killed me for the 5th time in 20 minutes i threw off my pants in rage. But when i heard that invasion sound a 6th time my rage turned to questioning... What was their build?"

Try guild wars 2. Very good game with lots of content and you can socialize without paying monthly.

Final Fantasy 14 or Black Desert Online, I suppose