If cheating on your spouse and having sex with people who aren't your husband, and giving him permission to do the same, doesn't sound ungodly then it's time for you to stick your nose in your bible and start praying because even most non-Christians know that's against the bible.
Truth is your husband is a douche bag that is looking for your permission to cheat because he's bored with you. And probably not just sexually either. There's a solid possibility he's into porn and that's where he got the Idea. Eventually one or both of you will get involved with someone else in a deeper sense.
Really this is nothing more than sleeping around. Except worse because you're also cheating on your spouse. Marriages like this have a higher divorce rate than other marriages. Cheating (beyond the physical) and jealousy are inevitable.
Not to mention that if your husband starts sleeping around, he'll be sleeping with others that sleep around. How long before and STD finds its way into one, or both, of you?
It saddens me that someone calling them self a Christian can't figure out if cheating and sleeping around while married are sins. Lust is a sin, and you never even touch the person.
So if you want to destroy your marriage, live suspicious and jealous until your marriage falls apart, all the while plainly and obviously going against God's word, then go for it. But when your life falls apart just keep in mind this is what you agreed to and you'll have to take responsibility for your choice to allow this. And all the consequences that Will follow.