Stakes

"Boys think with our nether head, so we're very prone to tell narratives of sex experiences in a good light.

Girls (while they *are* hornier than boys) think more with their upper head, so they're simply fewer likely to give an early sex experience a positive narrative."

No.

Girls simply and objectively have more to lose.

For a boy, impregnation with no commitment is a net gain - he gets to spread his genes, and she and her birth family end up paying for children.
For a girl, it is a net loss - she ends up raising her baby without paternal support she´d have liked to have as a breakeven point for the baby.

Even when there is no actual pregnancy and it is even specifically ruled out, the risk and action in direction of un(der)supported motherhood is what is instinctively perceived as negative. Girls are instinctively desirous of building up a safe nest, in terms of credible commitments... and if she is giving away sexual favours without getting adequate rewards in terms on nest-building, she´s likely to feel it as a loss.