most vaginas (a statement from unplanned polls of people i have spoken to on the matter) need to go through an experience period (usually approaching six months of regular activity) before they can experience pain-free sex. that is all
My similar meaningless annecdotal evidence suggests to me that the key factor is nothing to do with an "experience period" and everything to do with a) being relaxed about sex, b) actually wanting to have sex with their partner and c) having their partner have some skill at warming them up first. Nearly all the girls I know who describe their first sexual experiences as painful, didn't really want to have sex with their partner - those who don't; did.
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