hmmm I've enjoyed reading everyones post (I haven't been on for a while and just sat down and read this whole thread). I have been contemplating this same question recently - my finace and I are getting married in two months and our girlfriend will be right there beside us, and hopefully we will have a ceremony for all three of us when the time is right. We have been bringing this lovely lady to family events over the past year, and certain members of the family know and so far most reactions have been very negative -- for example my mother said that my vows will be nothing but lies, that I am an adulteress. And my grandma who I am /was (?) very close to said that it is just wrong, and there is no chance for it to end well. However they are all still coming to the wedding, so I guess they are able to come to terms of some sort.
Most of my friends know. I as well will not lie about our relationship (or any other aspect of my life) but I don't flaunt it either. However, when we are out in public we all hold hands and display a variety of PDAs, so I suppose that's flaunting to a point...
Anyhow, I have found that very few people have come up and been horrified. I have had lots of "hmmm I couldn't do that" and "well as long as you are happy" and best of all I have found a number of friends that I already had who happen live similar lifestyles!!
Sorry for the long post - short version, I can't lie - I'm no good at, and I want to live my life proudly. I also wouldn't feel right misrepresenting my SO's significance in my life.