He is not a good husband. His demands that you see and sleep with someone else against your wishes are not those of a good father either; do you have daughters? Do you want them to learn this behavior, come to know this is what "men" are like?

I would not see the other man, even socially, much less sleep with him. You need to set clear boundaries for yourself instead of "settlling" for what these immature, selfish men want for themselves and you and hurting as a result. Get help for yourself and someone to help your husband grow up and become a good husband and father before it is too late.