Glad to hear that you haven't lost your friends over a surprise in the scene...
Consider the trust and affection they have for you as a friend. Then consider the scene- a hot MFM that you have really found enjoyable.
You list your profile as 50/50, and you self admittedly love a hot cock.
As others have said, this dude shared his wife with you- and despite initial surprise to some direct physical contact- I think maybe you should kinda expect some contact when you're in that close scene.
Everyone has a "script" that runs in their head and certain things turn us on and some things shut down the eroticism pretty fast. One of the fastest routes to flacidville is fear and repulsion. So ask yourself: do you trust your buddy? Does he repulse you? Do you feel that he wants you to feel good and safe and sexually fulfilled?
My point is, as LPSGers repeat OVER and OVER and OVER: sexuality is fluid. What we like can change, and what we are willing to do can gradually change over time. I think it's not bad advice to mentally commit to opening yourself up to your buddy in the next tryst: either at the desire of his wife, or him- or..... YOU. If you love them and cherish them (as friends) then just relax and trust that they both want you to be happy and enjoy the scene to the fullest.
Chadstallion said you should let your bud suck you off while you go for sweet pussy. I actually fully agree- why not see if it works for you in a trusting scenario with people you like and who like close sexual erotic time with you? If you lose your hardon, or you can't go there you'll know--- just turn to your buddy, laugh and giggle a bit, and say, "it's not working" and he'll probably get it. Then you have comfort knowing that the reason for the rejection isn't personal rejection, it's truly a matter of biology- and you're wired to stay focused on complementary parts
Relationships are compromise- and growth and evolution means that nothing remains the same- either we go forward and get bigger and wiser and more experienced, or we shrivel up and the muscles atrophy. Be willing to give of yourself to those who you are calling your sexual playmates- and trust yourself and them enough to make your body vulnerable. You just might surprise yourself.
And remember bud: if your sexual object choice is based upon what cues you have learned to put you on the road of inevitability, your buddy's girlfriend is in the mix.
KICK BACK AND ENJOY IT ALL buddy!
Let us know how this goes the next time- I think a lot of us can learn from where this goes with the three of you!