It isn't really down to anything American. What you're doing is asking the other person to make a lifetime romantic commitment to you. You're already prepared for that kind of commitment; that's why you're asking. When they say "no", that's pretty much it for the relationship, because it's demonstrated a clear imbalance in how the two of you feel about each other.
That's what refusing a proposal says. At best, it's saying that you're happy now but you want to leave your options open for leaving at any point in the future with a minimum of fuss. That's not really the kind of sentiment that leads to your relationship continuing on.
Frankly, I don't understand people who have their proposal refused and don't break up.