I've had the opportunity once and passed. I just couldn't do it. The chick was smokin', but it was my best friends girl and he was the second guy. I felt, and still feel, that I know him about as well as I want to. I really don't need to know what his penis and fuck-faces look like.

Edited to add that it was an oppurtuniy for a threesome, but she specifically said before hand that she wanted to do a D.P. Even if it there was no D.P., I still would have passed.