I misunderstood i thought you just opened up after meeting this new guy, thats why it sounded planned to me, my apologies.

But yes even though you both agreed to an open relationship i think once you both start sleeping around it will end the relationship. At some point one of you will refuse sex with the other and an insecurity will pop up, do they enjoy another person more, am i not good enough anymore, do they get more outside so they dont need me, am i just an emotional support partner? Its like having a threeway and your partner showing more attention to the other person, it will eventually rub you the wrong way and cause issues.

I honestly think that if a relationship isnt open at the beginning then it should stay closed, more times then not if you open up later in a relationship it ends badly. Just my opinion, never really kool with the thought of my partner fucking someone else or building intimacy with someone.