You cheated behind his back and know have reached the point of asking for an open relationship. No offense, but shouldn't the conversation come before the affair?

Open relationships rarely work. For one their built on trust, both partners set clear concise boundaries and trust that one another will follow set rules and also keep all relations outside the core relationship nonemotional. You've already cheated behind his back, which has already ruined whatever trust he had in you.

Aside from that, if he considers himself monogamous then your clearly at an impasse. No amount of pushing will change his mind, especially after your affair. If you can't remain faithful, and refuse to remain monogamous and work on the marriage then the only choice left is divorce.

If you were responsible and truly cared for your husband you would have broached the topic of opening up the marriage before committing your affair. He would have said no, and you would have decided whether staying would be in your best interests. Instead you broke his trust and forever scarred your relationship, you asking for an open relationship now is just a kick in the nuts. Divorce and let him go, he deserves better then you.