I think the best thing is to introduce some fantasy play / porn first to help normalize the idea. Have her go down on you while you use a dildo on her. Say you think it would be hot to watch porn together or have her watch some porn on a tablet while you go down on her. Initially you can let her just browse around, then at some point, steer her toward some MFM. At first, just do it and let her enjoy it, don't talk about the cock being "another man" until later on, once she's used to the dildo play. My wife likes to get DPed with my cock and the dildo and gets turned on now when I talk dirty, asking her if she's a slut who needs two cocks and she'll say things like "I want to feel them both cum in me at once." It took quite some time to get here though...
I straight up told my wife some years ago that I thought it would be hot to see her with another man and she got super upset, cried, thought I secretly wanted to be with another woman, I didn't love her enough, etc... but then the next day she was ready to make up and we talked, I explained why it turned me on and then she was horny and we had sex lol.
Some wives are open to the idea right off or say they aren't but don't get upset. I got the more challenging end of the spectrum and so its been slow going. What's funny though is all along, I've known the idea turns her on, she just initially couldn't admit it to herself, then me... was worried it wasn't normal, etc. I think for a long time she also had this fear that no matter what I was saying I wanted, that maybe it was some kind of trap or I'd get upset even if it was my idea.
She hasn't been with another man yet but now she's pretty into the fantasy, especially after some drinks. She loves sucking my cock while I use a dildo on her or, what was really hot recently is her fucking a dildo suction-cupped to the headboard while I watched, then she went down on me. She even has asked if I want to do that again. Last time I had her watch porn on my tablet while I went down on her, I handed it to her with a video ready where a guy watches his wife get fucked, then the other man cums in her and leave, and she turns around so her pussy is over her husband's cock and lets the other man's cum drip out onto his cock, which he then uses to masturbate before fucking her and adding a second load to her pussy. She watched a number of videos while I went down on her and after my wife came, I asked her which video she liked the most and she said the one with the wife dripping cum on the husband's cock. 😏
So... I've gone from my wife being offended and upset to her being really into the fantasy and I think maybe ready to take another step... but it took quite some time to here. Anything is possible though with patience and communication. A big thing for my wife earlier on was seeing me get hard and cum during fantasy talk about her taking other cocks, which was indisputable for her that I was really turned on by this idea. I remember her saying something like "wow, you really do like this."
Part of the problem is, with some wives, just explaining things with words isn't enough - they have to be shown and see how you react, before they're ready to accept it and let themselves get into it.