This just goes to show how different people can have really different experiences. James and I both adore threesomes and have never had an awkward or unpleasant one, even before he was interested in having any sexual contact with another guy. Maybe it's because we've always done them together and so it's really just fitting the third person into the mix - something that for us at least has always been somewhere between great and stellar. We refer to it as Hot Tetris.
For us, threesomes break up old sexual patterns that tend to happen when you've been together a long time, and with MFM in particular, I'm the one who is leading the way - pretty much the complete opposite of porn scenarios. It's a little slice of heaven (with a woman too) and I can't think of any sexual thing that I enjoy more. But, I also think that our experience isn't necessarily the typical one, and I agree with you about DP. We've only ever gotten that to work with one particular partner, but it was fun!