I'm in a (theoretically) poly relationship and I agree with @TheSilkWorm .
(Theoretically because we're both too busy with other life for other partners ATM, but we're both happy with the concept and it has been a thing in the past)
If you go into a relationship being upfront about wanting a poly/open relationship right from the start, that's fine, people can accept or decline on that basis.
Trying to unilaterally change the whole basis of the relationship after years of monogamy and presumably monogamous marriage, not ok.
One partner is not keen - not ok.
Having discussions over time to explore it and both people are curious, positive and want to try, and have great communication about what it could mean for their relationship, then possibly ok.
Also, @hadenough24 , what is his response if you ask about you having other partners as well? Many of the so-called "open" husbands when I hear about this scenario, they seem to believe that the "open" should only apply to them...