I have had the good fortune to have been part of a M-F-M experience with a good friend & her husband. As several people have commented, I did feel a bit dubious about even Seeing another guy nude, never mind maybe having to touch him; like Lixychick and her hubby though, it was a totally spontaneous thing, and as both of us (guys) were totally into my friends' pleasure, if we did touch each other we didn't give it a (negative) thought.
In fact, we all liked it SO much, we did it again a few weeks later, but planned this time - and it was even better 2nd time around!

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The selfish, they're all standing in line
Faithing and hoping to buy themselves time
Me, I figure as each breath goes by
I only own my mind