Not sure i am the best to address this as this has not happened to us yet. Others have told me that it takes a long time for the woman to accept that this is something that the husband wants and it wont damage their marriage.
I think we have finally got to that point (after more than two years, I forget how long now), I found the main obstacles to overcome were firstly , that she thought I didn't want her and wanted to get rid of her to another man, and secondly, its just an excuse so I could have sex with other women. I had thought I had nailed those some time ago but actually it is only been recently that I realise she has fully accepted that neither of those were true.
This doesn't mean that she is ready to do it, but it does make our fantasy role plays much more free, and she often initiates thoughts of sex with a stranger.
Only this week, I had been away for a few days, and we didn't have time for full sex, but I was fondling her tits, and started to talk about another man touching her knee when we were out. Well she was so turned on, she didn't have time for that and went straight to talking about his cock inside her! Didn't take long for her to cum.
Still I don't know how to go further, generally she is quite shy, and I know pushing things will backfire, I feel she needs to make the decision and so I am just supportive and confirm that I l want her to do it, and will love her whatever.
Maybe it will never happen, but I am encouraged by her obvious excitement when role playing in bed. One other thing, I am sure most men in our position have anxieties and doubts, but I am careful not to let those show and always show a positive attitude to it.
I also would like to hear from those with a more reluctant wife how they moved on.