Sorry, but reaching a bit on this one... Contraception has been part of the human experience as long as humans have been able to figure out ways to use it. I forget the name of the plant, but there was a plant that was literally harvested to extinction because it was known for its contraceptive properties: http://www.jstor.org/stable/29774642?seq=1#page_scan_tab_contents alligator dung, honey, 1/2 a lemon and various other techniques have been documented as used for such purposes all the way back to at least 2000BC.
Sure the pill may be far more medically reliable, but birth control is hardly something that is new in the history of marriage and human sexuality.
So has adultery. That does not make it legitimate. Contraception within marriage denies the openness to children as one of the primary ends of sex and marriage. It is a revolt against our surrender to the will of God, seeking only the pleasures of marriage without the responsibilities. It is also a invitation to non-marital sexual relations that seek the pleasure of sex as an end in itself. If married couples can engage in sexual activity without the purpose of begetting and raising children, why cannot others?