I can see a difference between "no obligation to intend the end" and "no obligation to not exclude the end".  Every time I eat I don't necessarily have nutrition on my mind.  My primary thought might be to enjoy the taste of the food.  But not explicitly intending nutrition is not the same thing as intentionally avoiding nutrition.  e.g. eating something that has no nutritional value whatsoever with the sole intention of enjoying the taste.

Mith, you are correct, but only in the sense that the ACT is lawful and duly ordered.  Where you are incorrect, is that the INTENTION to avoid conception is NOT LAWFUL, as the sacrament of matrimony has as its purpose the God-ordered duty to "increase and multiply".
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Periodic abstinence is only allowed for grave reasons, which in pre-V2 days required the permission of a priest.  Those who practice it without permission and without a grave reason, are wrong.

On the other hand, if the act be limited to the sterile periods insofar as the mere use and not the right is concerned, there is no question about the validity of the marriage. Nevertheless, the moral licitness of such conduct on the part of the couple would have to be approved or denied according as to whether or not the intention of observing those periods constantly was based on sufficient and secure moral grounds.  The mere fact that the couple do not offend the nature of the act and are prepared to accept and bring up the child which in spite of their precautions came into the world would not be sufficient in itself to guarantee the rectitude of intention and the unobjectionable morality of the motives themselves.

The reason for this is that marriage obliges to a state of life which, while conferring certain rights also imposes the fulfillment of a positive work in regard to the married state itself.  In such a case, one can apply the general principle that a positive fulfillment may be omitted when serious reasons, independent from the good will of those obliged by it, show that this action is not opportune, or prove that a similar demand cannot reasonably be made of human nature.

The marriage contract which confers upon husband and wife the right to satisfy the inclinations of nature, sets them up in a certain state of life, the married state.  But upon couples who perform the act peculiar to their state, nature and the Creator impose the function of helping the conservation of the human race.  The characteristic activity which gives their state its value is the bonum prolis.  The individual and society, the people and the state, the Church itself depend for their existence in the order established by God on fruitful marriage.  Therefore, to embrace the married state, continuously to make use of the faculty proper to it and lawful in it alone, and, on the other hand, to withdraw always and deliberately with no serious reason from its primary obligation, would be a sin against the very meaning of conjugal life.

There are serious motives, such as those often mentioned in the so-called medical, eugenic, economic, and social "indications," that can exempt for a long time, perhaps even the whole duration of the marriage, from the positive and obligatory carrying out of the act. From this it follows that observing the non-fertile periods alone can be lawful only under a moral aspect.  Under the conditions mentioned it really is so.  But if, according to a rational and just judgment, there are no similar grave reasons of a personal nature or deriving from external circuмstances, then the determination to avoid habitually the fecundity of the union while at the same time to continue fully satisfying their sensuality, can be derived only from a false appreciation of life and from reasons having nothing to do with proper ethical laws.

[Affirmative precepts] do not bind for always, although they are always binding; but they bind as to place and time according to other due circuмstances, in respect of which human acts have to be regulated in order to be acts of virtue.

-- St. Thomas Aquinas, ST II-II, Q. 3, a. 2.

An affirmative precept commands us to do something, and a negative precept forbids us the doing of something. The first is called affirmative, because it is delivered in the affirmative: do this or that. The second is called negative, because it is commonly intimated by the negative: do not this or that. The difference between them is, that an affirmative precept obliges always, but not upon all occasions; but a negative precept obliges always, and upon all occasions.  For example, the affirmative precepts of Faith, Hope, Charity, &c. oblige us always, so as not to do any thing contrary to them, but yet we are not bound to practice these virtues upon all occasions, but only upon certain occasions when these precepts oblige us. But the negative precepts of not hating God, etc., oblige us always and upon all occasions, because there is no time nor circuмstances in which the acts of hating God or our neighbor are not criminal.

-- Burke, Thomas, Bishop, O.P. (1752). A Catechism: Moral and Controversial.  pp.189-190.

A positive law ceases to be of obligation in a particular case if it becomes hurtful, or if it cannot be observed without serious inconvenience.

-- Slater, Thomas, S.J. (1925). A Manual of Moral Theology for English Speaking Countries (Vol I).  p. 69.

And sometimes that purpose is to conceive.