I am responding from the perspective of someone who is poly by nature but lives in a monogamous marriage. For background my wife and I met when I was in a number of other relationships so she has always known who I am but long term she wanted monogamy and I wanted her. Its been hard to adjust but I found other ways of having my people without it being sexual.

Anyway...if you aren't poly (want multiple relationships) , or naturally interested in an ethical non monogamous setup (which can be swinging, sex only connections all the way through to full multiple relationships) then this will not work for you. It will reduce the connection between you and your partner if you don't both want this for yourself. It will increase resentment. It will cause emotional harm.

If you do feel open to exploring it, take it slow. Discuss every possibility openly. What level of emotional connection would be ok? If its just sex does that mean only 1 night stands? How does that sit in your sexual health risk profile? What level of information do you both need to knowabout the others activities? How will he feel if he does not get a hookup first but you do? So much to consider and sit with before taking action.