When Is "Family Planning" Moral?

So... let's follow-up on our discussion this morning. This question came up in the last five minutes (thanks, Ken!) and we talked about it some, but I think we wanted to develop it more. Here, again, is the Catechism section that we found on this:

2368 A particular aspect of this responsibility concerns the regulation of
procreation. For just reasons, spouses may wish to space the births of their
children. It is their duty to make certain that their desire is not motivated by
selfishness but is in conformity with the generosity appropriate to responsible
parenthood. Moreover, they should conform their behavior to the objective
criteria of morality:
When it is a question of harmonizing married love with the responsible transmission of life,
the morality of the behavior does not depend on sincere intention and evaluation of motives alone; but it must be determined by objective criteria, criteria drawn from the nature of the person and his acts criteria that respect the total meaning of mutual self-giving and human
procreation in the context of true love; this is possible only if the virtue of married chastity is practiced with sincerity of heart.156
156 Guadium et Spes 51 § 3.

Here's another section to consider:

2370 Periodic continence, that is, the methods of birth regulation based on
self-observation and the use of infertile periods, is in conformity with the
objective criteria of morality.158 These methods respect the bodies of the
spouses, encourage tenderness between them, and favor the education of an
authentic freedom. In contrast, "every action which, whether in anticipation of
the conjugal [married] act, or in its accomplishment, or in the development of its natural consequences, proposes, whether as an end or as a means, to render procreation impossible" is intrinsically evil:159
Thus the innate language that expresses the total reciprocal self-giving of husband and wife is overlaid, through contraception, by an objectively contradictory language, namely, that of
not giving oneself totally to the other. This leads not only to a positive refusal to be open to life but also to a falsification of the inner truth of conjugal love, which is called upon to give itself in personal totality. . . . The difference, both anthropological and moral, between contraception and recourse to the rhythm of the cycle . . . involves in the final analysis two irreconcilable concepts of the human person and of human sexuality.160
158 Humanae Vitae 16.
159 Humanae Vitae 14.
160 Familiaris Consortio 32.

In our conversation, we narrowed the key point of this passage to whether a decision was based on "selfishness" or not. Do you still think that's true? What is selfish and what isn't? What does "for just reasons" (section 2368) mean? Who decides?