CathInfo.com

It is my understanding that holding off having kids until a predetermined milestone or date, does not make the marriage invalid. Because, technically, you are open to having children. However, not to be open for kids would definitely make it invalid.

I have a very hard time understanding how error about the nature of marriage (exclusive and indissoluble) does not ipso facto invalidate a marriage, or at the very least a sacramental one (as opposed to a natural marriage between a baptized spouse and an unbaptized one, or two unbaptized spouses).  To confect a sacrament validly requires the intent to do what the Church does (facere quod facit ecclesia), and the Church does not intend to confect a sacrament that can be dissolved by any earthly power.

I know there is back-and-forth on whether a sacrament is confected when two Protestants marry.  The modern concept is that they do have a valid sacramental marriage, and if this was seen differently in the past, I would be interested to know the particulars of that.  And as to the Orthodox, well, they would come back and say "we don't speak of sacraments as being valid or invalid" (or something like that).  They've always got an answer.

An error or mistake of law concerns the nature or essential object and properties of the matrimonial contract, as in the case of ignorance. Thus, a simple error regarding the unity or the indissolubility or the sacramental dignity of marriage, even though the motivating reason for entering into the contract, does not invalidate the matrimonial consent. A simple error is one which remains in the intellect without a positive act following in conformity with it. Thus, as the essence of matrimony has inseparably attached to it is essential properties, in consenting to marriage as it is in itself, consent is thereby also given to its properties of unity and indissolubility. To know a thing with all its properties and to will a thing differ; one can simply will a thing as it is in itself and not know very well its properties or be mistaken about them. Thus an error about the essential qualities of marriage does not necessarily invalidate it. The general intention to contract marriage as instituted by God prevails over the error. Although many consider marriage to be dissoluble and not sacramental, e.g. protestants, Jews, infidels, yet they normally will to contract marriage as it is. They probably would positively exclude these properties if they were later questioned about them, but they did not actually exclude them at the time the consent was given.