i wouldn't have one. i was talking to a guy once and he was in an open relationship, i had a conversation w/ him about it and he told me the reason they had one was because his boyfriend doesn't enjoy sex (with anyone, never has) so the boyfriend gave him permission to have sex w/ anyone..
but then it turned out the boyfriend was actually super jealous and sad about it secretly, he just gave permission bc he didn't want to let his partner down sexually
if i had an open relationship i could see myself being like that boyfriend.. secretly jealous and sad but telling my partner i'm fine with it to keep them happy