It's laudable that he is honest with you about what's going on. It seems that he is, for the moment, also respecting your boundaries and giving you time to work out your feelings. He is in a relationship, however, that is a secret (she's not honest with her husband) which could undermine the whole dynamic. The demands on communication, honesty, and patience are sufficiently high in poly relationships that her deception is easily a problem for all.
It seems like a hard thing all around. I wonder if how your husband would react ot requesting more time?
Male, Straight, Poly
“Instead of getting better and better at avoiding, learn to accept the present moment as if you had invited it. And work with it instead of against it. And making it your ally rather than your enemy.”