To me, this is a question of sorting out the priorities in your own mind... the competing priorities are -
* being open about who you are to those who are close to you
* Having a functioning relationship with members of your family
Your concern is that these two are mutually exclusive, and this is supported by some solid background evidence.
I have faced something similar, and we came to our own agreements about what was the most important - we ended up doing different things for different groups. I really don't think that it's something that other people can answer for you, because this so completely depends on your own priorities in this regard.
Another way to look at this: if you go with the first, there is a possibility that the second will still be in place. If you go with the second, there is no way that the first will be in place.
Take a look at the worst-case scenario - the parents on both sides judge you and won't have anything to do with you, and tell their friends what you are doing (I *did* say worst case) - how enmeshed are you in their lives and circumstance - how would this affect you? Yes, it would be a massive shame, but in terms of living your life?
I don't think there's a magic wand, here - it's about thinking about priorities, then, once you've decided to do it, working out the most diplomatic way of doing it.
Oh, and while this issue is still up in the air, I would avoid any actions which could be considered to be inflammatory - this would include a big ceremony like a handfasting. Either get this sorted out before you do it, or postpone until until things are settled.
"Listen, or your tongue will make you deaf." - Native American Proverb