Not being able to afford birth control can lead to a much bigger burden for the person and the government supporting them in the long haul. If you want to save money you don't pay for the child that the person having it can't afford after it's born, you pay for it before it's conceived.
It makes no sense that Viagra and Cialis have no co-pay or very low co-pay but birth control has a high co-pay. I was a struggling College student in NYC not that long ago, and my birth control cost me $50! I worked a full time job while going to school full time, and I still couldn't pay for it. I ended up not taking it anymore. If this is the case for most women, then they'll end up with a bay if not careful. A baby costs more than birth control. Why is erectile dysfunction more important than preventing something that could cost thousands, and possibly end up being abandoned?
The World Health Organization, the National Institute of Health, and the American Medical Association all have released statements indicating that contraception is one of the key factors in women's health and providing both longer life expectancy and better standard of living for women. WHO has also endorsed what many sociological studies have also found- that readily available birth control is vital to women's advancement and independence.
In addition, pregnancy and birth cost the insurance company (and thus all the insured as the costs are passed on) vastly more than subsidizing birth control would.
The cost of having a baby is a lot more than paying monthly birth control. If insurance would cover the cost of birth control, then that small amount they are out would compensate for the big picture, pregnancy. It can cost several thousands of dollars to have a baby, and a co pay is $20.