I'm a big fan of sharing, but in my little world, nobody brings anybody home without us having talked about it first, regardless of whether it's another man or a woman. As AA pointed out, the potential for disaster is huge, not to mention hurt feelings, jealousy, insecurity..
I wasn't surprised by the majority of responses to this question.. I've noticed most guys have no problem thinking of, or actually indulging in, sharing their wife/gf with another woman whereas the thought of sharing the same wife/gf with another guy is generally upsetting. Whether it's as StuartD suggests, and men don't really consider lesbian sex to be 'real sex', I'm not sure, but as pornodoggy discovered, most *women* consider lesbian sex to be 'real sex'. LOL
So, in my eyes, whether I'm with another man OR woman, I'm sharing myself sexually with someone other than my partner. And the same reasoning applies to my partner. Whoever it is, we're bringing him/her home together.
Great question, Ronaldo!