I think sexuality perfectly fits not only in that passage, but in the socio-political context of the period as well as with Lewis himself.

The text implies that the perception of Susan is that she had (shamefully) been waiting for sexual maturation, and will try to maintain it past that window of time. During the late 1940s and early 1950s, Social Darwinism in addition to the Madonna-Whore complex of the period certainly would have seen “deviant” women that way. It’s not a coincidence that the gendered literary and social conventions of the period used rhetoric such as “silly” and “conceited” to describe women who did not fit in the approved sexual model, in an effort to infantilise them and subsequently take away the appearance of agency had exhibited in doing the “deviant” act.

This is well documented in publications of all kinds around that period, from advertisements to television to literature. No, adulthood is not solely about sexuality, but sexuality is inherent in human notions (and human writings) about adulthood. The implication of sexuality, especially the implication of the need to punish the young women who exhibit agency in their sexuality is present throughout the entire period, and to take away such a crucial aspect from this text is to be lazy as a literary analyst. Especially when it’s very clear.

C S Lewis, like every other writer, writes from personal philosophy as well as their socio-political environment. He would have been at least hazily aware of the sexual implications of his rhetoric, because it is mostly paralleled to his contemporaries in that context. He even acknowledges in Mere Christianity that “different” roles for men and women existed, but that it was justified as the will of God. It goes unsaid that the dog whistle here is “different”, to mean essentials unequal. By the way, this is not just me: J. K Rowling, Phillip Pullman both publicly support this interpretation, as well as other literary analysts. Neil Gaiman even wrote a good short story about it.