I know a couple that has two children. They have never had sex without contraceptives. Both are the results of contraceptive failure.
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I am not going to believe that you have been a witness to their every carnal act, so I think what you really mean is that they SAY that they have never had sex without contraceptives, and you believe them.
I find no difficulty in believing that you believe them, and only slightly less in believing that you heard them say this. That is, if indeed I believe that they, in fact, exist. There is always the possibility that both of them are figments of your imagination.
I know this: when I was growing up, there were no BC pills. There were condoms, diaphragms and spermicides. I grew up in a small, mostly Protestant town where people generally thought two or three children was the ideal, and in fact, very few people had more than three.
It is conceivable that Bristol Palin's unborn fetus was the result of one, two, three or perhaps even four failed BC devices plus the use of the rhythm method.
It is also possible that she was fecundated by an alien named Gorn from the Planet Koozbane in her sleep during one of the rare Aurora Borealis abductions. Possible, not probable, but possible.
However, the greatest odds are that said pregnancy was the result of the non use of any BC device at all.