Speaking as the "mono" partner who found out months down the road that Easy was in love with Asha before he talked to me about poly, it doesn't make it easier to accept. I spent a long time feeling used and that they manipulated me in order to get what they wanted. I also spent too much time not feeling wanted for myself, like Asha had used me to get to Easy and I was extra. There was actually a period of time where I had a bag packed and I was ready to leave so that Easy and Asha could be together.
I don't know if it would have been easier if Easy and Asha had talked to me about how they felt before I agreed to be poly, or if I would have felt blackmailed. I think you need to talk to your husband about being poly right now, before you go any further with the other guy. You can afford to be friends with the other guy for a while longer for the sake of your marriage. If you can't afford it, then maybe you should take a closer look at whether you still want your marriage.