We live in a society that now says you're stifling and controlling if you interfere in your spouse's choice of friends and social activities. Because of this, I find sooo many people toleratring their SO do things with friends of the opposite sex (like going to a concert alone or meeting for drinks alone) that in years past would never fly because it was socially taboo.
Just because society now says it's "okay" for men and women to be friends, even if they're married, doesn't mean that it won't cause damage to your relationship. People are totally asking for trouble when they allow others into their marriage in this way. Biology can often over ride reason. And in the case of male/female relationships it usually does.
I don't know too many married people with friends of the opposite sex who eventually didn't have a crisis somewhere down the road. Ultimately, choices will have to be made. The spouse is supposed to be your best friend, confidante, and lover. There is no need for an outside source to fulfill these needs. If you have a good marriage, usually your friendships take a backseat. If they're (friends) are pulled together, they have a life and marriage of their own and it balances out.
For those married couples who do indulge in outside friendships, it may seem like it's working fine and everyone's okay, for a while. But when one has a "friend" and the other doesn't, there is an unspoken debt accrued in the relationship. When crisis strikes, there will be accusations and blame on the part of the spouse who "allowed" the friendship. They will punish you for your choice by either indulging in a "friendship" of their own or by tightening the reigns so to speak and control you in ways you couldn't imagine.
Over time, the allower will become jealous and filled with mistrust and hate for the other. Even if they are having an affair of their own. There is no way to fix this type of breach to a relationship because from day one, you told each other that you weren't that into each other in the first place because the allower said "knock yourself out and be friends, I don't care." and the other made their "friend" their new partner..and their real spouse the third wheel.
This isn't about trust the way you see it. It is about breach of contract. You and your husband are playing chicken with your marriage by telling each other "you're not fulfilling enough for me...I need more from x over here." Perhaps for your husband, he has alterior motives such as getting the go ahead from you to find a friend of his own. Naturally, since you have your friend, he expects you to say it's okay for him to do the same becasue If you had one first, you can't point fingers or question him when he has one of his own.
I say if your hubby isn't enough, and you don't share the same interests, perhaps marriage counseling is in order. No friend is worth destroying your marriage over.