A quick rule is if you can substitute "he/she" then use "who". If you would need "him/her" then use "whom". Although a straight substitution isn't possible in your original sentence, look at it like this:
How did you decide to marry him?
If you don't date a man, how will you decide whether or not to marry him?
You wouldn't use "he" in either of those sentences instead of "him", therefore "whom" is appropriate, not "who".
I may be wrong but I think the following may also be relevant:
To whom will you be married?
"Who" becomes "whom" after a preposition.
Student or Learner