Well, I currently don't have a GF I am active with, but my wife has a BF and we have done the threesome thing, and it was pretty hot. But we are both very straight, and only share a friendship and the love of the same woman. So that combo was all about her.
And while I enjoy seeing her enjoying him and him pleasuring her, there isn't a single thing I find sexually attractive about him or any guy!!! But we can still do the MFM thing fine... ;-)