Thank you for being a light that shines on reality about this subject.In my conversations with couples the 2 main objections have been her fear that he wants to be with other women so giving her license to be with other men is his first step on that path and her fear that she will alter their relationship for the worse if she is with another man. That second fear is generalized usually without a specific known outcome.
There's no one way to open this door. Every couple navigates the process their own way. But I do suggest getting out of town. Don't do this in your backyard to begin. And not with friends. That's too high risk. Plan a getaway and tell her it's your gift to her. No strings attached, no reciprocation expected. It happens one night and it doesn't need to happen again. Give her the space to be carefree and the security to know she is in a stable relationship before and after the experience.
When I hear advice that focuses on the male half of the couple that sounds like fantasy talk. Real advice focuses on her. Her safety, security, comfort, etc. And then you open the door and get out of the way. She either walks through or she doesn't and you respect her decision either way.
So many of the posts here focus on what the guy wants his wife to do. The last time I checked, I was the one taking my underwear off for sex, not my husband. So, let's look a little harder at what I (the wife) want to happen.
From what I have done, being with a new guy is kinda awkward the first few times. Even before my underwear comes off, I have to like the guy. Getting to know someone takes a bit of time. I'm not talking months, but not being introduced and 10 min later going up to the room and laying on my back.