Given evolutionary theory, shouldn't there be, over time, changes to reproductive factors in men and/or women to circumvent birth control efforts?
Nature finds a way.
There already is. Sex without contraception is way better than sex with condoms or BC methods - side effects of the pill, patch, etc are usually stuff like "unwanted weight gain," "terrible skin," "zero libido," "difficulty achieving orgasm" - all things that make people want to go off it. For guys pulling out or using condoms doesn't feel as good as bareback creampies.