I bet you could put an end to this stalemate by suggesting turn-about is fair play. He might think twice about the avenues of kink he dares to explore, or you could find yourself in thrilling new territory. Either way, be careful what you wish for and even more careful with how you negotiate this sensitive topic. That's just my two cents, let's see what the cards have to say!
Okay, let's be honest. This isn't all about the sex, or the potential discomfort of it, because you're a little bit curious about toying with taboo just as much as your husband. Looks to me like you worry that you're disappointing to your husband in some fashion, that you're falling short in satisfying him, that you somehow aren't enough for your man. This fear and worry of yours is hinging on this one thing in the bedroom, but I'm willing to bet if you dug a little deeper you've been worrying about this in other areas of your marriage as well.
Your husband loves you, he trusts you, and he wants to share an intimacy with you that he's not felt safe enough to explore with other women. That's a beautiful thing! But what I see here is that you worry if you give up control in this one area your husband will take that as a green light to “coerce” you into other things that are outside your comfort zone. What you call coercion your husband calls communication. Get on the same page. What other areas of your marriage are strained, do you feel a lack of connection with your husband? What can your husband do to make you feel safe and secure in all areas of your marriage? These might the things that need to be discussed before you consider giving him the secret knock to your backdoor.