Yeah, Mike needs to get his ethics in order. As the person here who's versed in poly, and as someone who knows your marriage just went through a rough patch, the LAST thing he should be doing is encouraging you to be dishonest because he thinks your husband might not be ok with things!!! Just, really not cool, Mike.
Tell your husband asap, hopefully no harm was done. You don't want him getting it in his head that sneakiness is how you two roll, so it's ok for him to be sneaky in the future, do you?
As for mono/poly relationships, sure, they happen, but that really only seems workable to me if the monogamous partner is naturally monogamous. If your husband wants to be with other people, and you deny him that option while taking it for himself... yeah, that's not gonna end well at ALL. Either he'll lie to you, as he's done in the past, or his resentment will poison your relationship over time.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.