Natural Family Planning is a part of the Church's teaching that I find very appealing.  I think Paul VI was right about the nature of sex and marriage in Humanae Vitae:
In the task of transmitting life, therefore, they are not free to
proceed completely at will, as if they could determine in a wholly
autonomous way the honest path to follow; but they must conform
their activity to the creative intention of God, expressed in the
very nature of marriage and of its acts, and manifested by the
constant teaching of the Church [10].

Some web-based resources