I agree with the "take it slow" advice.
If possible, dip their feet in the water by dating someone else while you're still "just dating" them, before it becomes a serious relationship and you get too attached. That would give you the chance to see how they actually deal with it when it's more than theoretical.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker