The core of what I ultimately told my parents:
It's made a big difference (compared to trying to reason with them, explaining my decisions). I believe the reason why it has been working so well so far is three-fold:
All that I ask from you is that you sincerely pray to know whether or not you can trust me and how you should treat me (i.e. whether or not you should treat me and my future male spouse as you would treat any of your other children and their spouses). I will respect and honor whatever choice you make just as I hope that you will respect and honor the choices I am making.
- I make no threat to their beliefs;
- the focus is on me as their son and how I'm doing in life instead of on a principle; and
- I stand as an example.
Debating principles, I believe, is futile. Their stances will be sufficiently challenged as they watch me walk the path I'm taking. Therefore, it is necessary that they are watching... and they won't watch if they feel like I will constantly threaten and question the beliefs they feel are holy.
And asking them to pray about me, whether or not they can trust me, really puts the focus on watching me, where it ought to be.
They also know that if my future husband is not seen as part of the family as is any other sibling's spouse then they will lose me. My mother would not (could not) stand for that.
My approach has been working so well with my parents that I'm considering sending a mass email to my siblings with the same message.
Granted, this is my family. Not everyone reacts the same. But it is the approach that I recommend. Take it for what it's worth.
I have more thoughts, but I'm just not organized in thought right now