That's because a woman using oral contraceptives is in a constant state of pseudo-graviditiy, so far as her hormones are concerned. That's also one of the major reasons why the rate of infidelity is so high among married women that orally contracept. Through most of their marriage, their physical attraction to their husbands is dependent on that specific hormonal balance. Then, they stop taking it when they "decide they're ready" to cooperate with Divine Providence and they suddenly realize that the physical attraction to their spouse is gone.
As if one needed any more proof that oral contraception is one of Satan's great triumphs, the attraction to less-masculine, even effeminate men while on oral contraceptives makes it much more likely that any unplanned children will express those physical characteristics themselves. Yet some wonder what happened to all of the real men, and why foppish boys, looking and acting like catamites, have become the sexual ideal.