Why do most girls end up getting pregnant on first try without wanting to

I don't want to sound mean or negative to single parents.. I know so many couples who have unprotected sex for first time and bam! Pregnant, once that happens relationship (most of the time) doesn't work out cause there was no planning involved, hence mom takes on both roles.

I started to think that why is it that couples who don't plan and don't stay together get pregnant and those who want to & have plans, takes forever to conceive... it's kinda frustrating to think about it.