Ask her to teach you a little bit on one of her vibrators or dildos today for practice so you have some confidence. When it comes time, use your hands and watch how he reacts. A pleasurable reaction from him should make it more exciting for you. Finally, have her help you first, the two of you do it together as she gives you instruction. As quoted above "It's just sex". Once you get the first time out of the way then it will be easier and more fun. It's just like the wives here who take so much time going back and forth and denying themselves sex with other guys, until they finally try it and love it.
You're just doing this to please her and him and you will find that it pleases you to do it. I can tell from your phrasing here and your forced complaints insisting that you have no desire to do it, that you really want to. It's ok. Really.
Just don't turn into one of the "sissy" types you see on here. Be a man and help your wife enjoy herself and help her lover enjoy her. Eventually you will be doing whatever she tells you to do because that's part of the fun. It might be a bit "humiliating" to you in your mind but that's ok too. It seems she wants to be a bit "dom" with you so try it out. The worst thing that happens is basically nothing. You suck a cock and then move on. Keep doing it if you enjoy it and stop if you don't. You probably love to watch her eat pussy don't you? There you go.