Your deacon has given you some rather garbled advice. Let’s see if we can break it down.
First, as to valid consent in marriage. The following canons apply:
Can. 1101 §1. The internal *consent of the mind is presumed to conform to the words and signs *used in celebrating the marriage.
§2. If, however, either or both of the parties by a positive act of the will exclude marriage itself, some essential element of marriage, or some essential property of marriage, the party contracts invalidly.
Can. 1102 §1. A marriage subject to a condition about the future cannot be contracted validly.
This canon can be applied to the essential element of children. In the vows you make you state you will accept children lovingly from God. Now, if you make a positive act of the will to permanently exclude children your consent of mind is not conforming to the words and signs of the vows.
So, yes, a permanent intent against children can render a marriage invalid.
HOWEVER… that said, your deacon’s advice to “have one and then use NFP” is completely outrageous. A child is not an object or a toy. You don’t “have one” for the reason he asserts.
What I would suggest is that you and your fiance do consider the Church’s teaching regarding the postponing of children in marriage. Contraception is never a moral choice so of course you would learn NFP to postpone a pregnancy if necessary. That said…
What the Church teaches is twofold:
(1) Each act of intercourse must be objectively procreative and unitive and the two cannot be separated. So, every time you have intercourse if you do so in an unaltered manner (no contraception) you are meeting this teaching.
(2) Periodic continence (NFP) can be used for a just reason. Your husband’s currently mental state of being “overwhelmed” by four children could be a just reason if you two discern together in prayer that now is not a good time to have another.
Basically, what you need to determine is whether your future husband really has a firm intent against children as in, “Nope, I’ve had 4 and I’m done. That’s it. Period”
Or, in his overwhelmed state is he really saying, “I’d like to have more but I don’t see how I can have another I’m so stressed with 4. But we can take it a step at a time and I’m open to talking about it and diserning it from day to day, month to month, year to year.”
If it’s the “nope, I’m done” then I’d say he has a permanent intent against children and you-- who have stated you actually want childen-- need to rethink your decision to marry him.
If it’s “wow, I don’t know how I could have another right now… but we’ll just keep praying and see how **we **feel in a few month/year/whatever…” then I would say he is not making a firm intent against children.
For your own sake, don’t capitulate to him if you really want children and he is firm against it. He’s not the man for you if that’s the case.