I think there is a difference between two people deciding this together and one spouse "realizing" this after years of marriage. The former makes sense. The latter is poorly veiled cheating.Who is WE and AROUND HERE?
OP: google ethical non-monogamy for the learning you can achieve on that topic. I would recommend a marriage counselor. I would specifically look for one who is not automatically anti-ethical non-monogamy. Whether it is for you and your husband, it would be helpful for him not to feel castigated out of the gate.
-- Coming from someone with decades of practice with ethical non-monogamy.