It is laid out in very expressive form, if a bit long, in the Humanae Vitae Encyclical Letter by Pope Paul VI. If you don't wish to read the whole text, lookup points 12 and 14.
http://www.vatican.va/holy_father/paul_vi/encyclicals/docume...
"It does not, moreover, cease to be legitimate even when, for reasons independent of their will, it is foreseen to be infertile."
Insofar as your claim "can't be dissociated from breeding" can be constructed as a correct claim, it requires understanding that to mean it cannot generally be actively sought to be dissociated from breeding, but even that is incompletely accurate as (per paragraph 16) "If therefore there are well-grounded reasons for spacing births, arising from the physical or psychological condition of husband or wife, or from external circumstances, the Church teaches that married people may then take advantage of the natural cycles immanent in the reproductive system and engage in marital intercourse only during those times that are infertile, thus controlling birth in a way which does not in the least offend the moral principles which We have just explained."
(The position laid in Humanae Vitae has often been criticized for being somewhat incoherent, which isn't really all that surprising given its history. See, e.g., [1])
[1] http://en.wikipedia.org/wiki/Pontifical_Commission_on_Birth_...
