Very pleased to see this post is generating some very thoughtful responses and sharing of experiences.
I thought I might update my own original post as it was quite some time ago and change is inevitable.
My husband and I, as well as his girlfriend, are openly poly and becoming quite active in our local poly community. My boyfriend doesn't volunteer that he's dating a 'married' woman as his family is extremely religious and doesn't respond kindly to any beliefs that are 'different'. He doesn't feel a need to explain and I support his choice to be as open as he's comfortable with. All of his friends, however, are aware of our relationship and have been 100% accepting and supportive.
My husband's mother was very concerned when he told her because when she attempted it many years ago with his father it didn't work! (That was a bit of a surprise to hear!). She remains skeptical, but still accepting of our life.
All of our friends have been absolutely wonderful, the most loving and accepting people in our lives. If not for them, I'm not sure if I would be able to bear the pain of being estranged from my Dad & step-mother. They did not take our 'coming out' well and haven't spoken to me in over 8 months, despite my attempts to reach out. I've stopped trying, it hurts too much to be rejected repeatedly. This wasn't the first time they have withdrawn from my life when they didn't agree with me so it's something that I've come to accept. My door is always open to talk and I still, perhaps foolishly, hope that one day they will want to at least try and talk, try and understand.
Aside from this one dark shadow, my life is happier and brighter than it has ever been before. I have wonderful partners and friends that bring so much love into my life. It's been worth it for me to live my life honestly, without apologies.