It was for that reason that I rarely made any commitment to an exclusive relationship. I didn't want to be drawn into cheating at any time. I was happy to commit to a relationship, but it was with the understanding that we could see other people if they knocked our socks off.
I can only think of one relationship where I agreed to exclusivity.
I can see where somebody would want to connect with someone new and then approach a spouse to discuss it. I'm not really convinced it's a healthy start to poly living by cheating first and then bringing it up. Seems like there would be some serious trust issues there.