Different relationship styles work for different couples. I could not be in an open relationship, especially because I am gay. I don't want to widen the chances of getting STDs. On top that, being exclusive makes the relationship even more special
If I wanted to keep my options open, then I would simply just do that!
And it is important to know what style works best for you. I agree with the user who said honesty is important. I'd rather my partner tell me he's into open relationships so that we could go on our separate ways, rather than he cheat on me.