Because of the separating effect of contraceptive use, they are not united. In the highest form of human commitment, and during the most intimate expression of that commitment of love, there is reservation, a holding back. You can have all that I am except my fertility. This you cannot have and so I will hold it back from you. With this reservation, the sexual union fails to be an expression of total giving, and therefore of total love.
Selflessness leaves the marriage, and selfishness enters.
If there is selfishness during the most intimate expression of marital love, then there will be selfishness elsewhere. This is because selfishness is like cancer. It does not remain stagnant, but grows and infiltrates every area of marriage, destroying the Christ-like beauty of a totally united love. One can also see why the Church teaches the necessity of every act of sexual union being open to the possibility of new life. Couples can only preserve the sacredness of the sexual union, and therefore of marriage, by ensuring the two ends of marriage are always present.