So he said he'd take it off the table ...but continued to cheat at that point.
Then got caught and now that its all in the open he decides to continue as everything is OK.
Cheater's are only interested in satisfying their urges ...many use poly as a cover to justify their behavior. That becomes really clear if and when the spouse then starts dating and they have a melt down.
If you want a mono life ...don't settle ....why settle for anything less. AND more importantly why settle with a guy who has no problem looking you in the eye and lying his ass off.
I suggest you both go to couple counseling to unravel the true source of the infidelity and how to deal with that. I'd demand a halt to his extracurricular fun until after some counseling you both determine the direction of your marriage.