What’s the holdup?
The problem is that some contraceptives alter the body's testosterone production--and not in a good way. This occurs for two reasons. First, the hormones in the Pill put the ovaries to sleep, halting ovulation. Conked-out ovaries can't produce testosterone.

Then your birth control renders the other 50 percent of your testosterone useless, thanks to the super-potent synthetic estrogen it contains. After you take each pill, your liver--convinced that you've consumed a potentially toxic amount of estrogen--starts pumping out a protein called sex hormone-binding globulin (SHBG). It works by glomming onto sex hormones (including estrogen, but also testosterone) like a mosquito to fly paper. As more of your testosterone glues itself to SHBG, less of it is available for your body to use. This "free" testosterone--whatever's produced that SHBG doesn't swallow up--partially determines your sex drive. In fact, a 2004 Boston University study found that subjects who reported the greatest sexual desire had higher levels of free testosterone. (Maybe that's A-Rod's problem.)