Those things take time, guys. If your wife had sex partner(s) before marrying you, you could give her a nice big dildo and when in bed tell her to pretend it's whoever her favorite lover was. You should then tell her how beautiful she looks while fucking him. You could name the dildo Jim, or Bruce or whatever the guy's name was. Be sure to compliment her at every turn about how sexy she looks while fucking "him" (the dildo) and how much she is turning you on. When she sees the enjoyment you receive from watching her being fucked, she may just graduate to having you set up the real thing for her and find a stud to fuck her good. Be sure to tell her that no matter what, you love her for doing this for you, and how much closer it will bring the two of you.
Work on it and you will succeed. Most women secretly want to do this, but they are afraid it will upset the relationship. It's upto you to constantly give her assurances that you will only love her more for doing it. Good luck, and enjoy your MFM. It's WILD! And you don't know what a REAL HARD-ON is until you see the one you'll have when you're finally watching her getting pumped good.