It's the nature of the verbs in this example that's causing the trouble.
Let's start by getting a different that-complement clause to contrast:
- I want to decide that the train arrives before noon.
(Using I will marry John, again involving the speaker, complicates matters unnecessarily.)
Want and decide are both mental acts. Which means they're very abstract and hard to combine.
Since they're both mental acts and they both take complement clauses, try reversing them.
Want can't take a that-clause, so this is ungrammatical:
- *I decided to want that the train arrives before noon.
but an infinitive complement is grammatically straightforward:
- I decided to want the train to arrive before noon.
though strange -- how can one decide what to want? Desires are emotional, decisions are rational.
There is a reason why want takes an infinitive complement with either A-Equi or B-Raising:
- I want to talk to the winner. [I want, I talk -- A-Equi configuration]
- I want her to talk to the winner. [I want, she talks -- B-Raising configuration]
These are both events, or at least states, that have not happened at the time of wanting.
Want can refer to the subject's desire for the event or state to happen, and it's normally presupposed that it hasn't. It seems odd to say
- I really want that portrait that's hanging over my mantel.
if in fact you already own it. But a common way to say that you are discarding it is to say
- I don't want it any more.
which suggests that there is another sense in which one (covertly) wants one's actual possessions.
Want is in fact closely related to have, in its sense of 'possess', as well as many other senses.
X, then what
A really wants is (for
A) to have
- I want that picture/a big wedding/a drink/a cigar/a baby/a long soak in the tub
- I want to have that picture/a big wedding/a drink/a cigar/a baby/a long soak in the tub
This relation has been around for a long time, as can be seen in the parallelism of some idioms with have, though not others
- have a tantrum/*want a tantrum
- have a convulsion/*want a convulsion
- have an operation/?*want an operation
Getting back to the strangeness of want to decide, we see it can make sense only if the subject has not yet made the decision. But the subject has clearly already formed a desire, which may or may not be independent of the decision.
Thus there is a conflict -- or, I should say, a vast uncertainty -- between the subject's mental states of desire and of indecision. This is the same reason why I decided to want sounds strange.