Let me go the long way around the pole on this one...
The shorthand claim for Evolution is "Survival of the Fittest" (coined by 1800s economist
Herbert Spencer, not Darwin), or more simply "Natural Selection."
The reality of it is having nothing to do with "fitness" it's about
Reproduction of the Survivors. For a species to continue to exist it has to 1. Survive, and 2. Survive long enough to Reproduce. "Fitness" is a consequence, not predecessor, of those conditions.
This puts Sex ahead of Survival as an instinctual drive. How many creatures will fight to the death to attempt mating? Consider the dying act of migrating salmon being eaten by bears is often spreading their eggs or sperm in a last attempt to further the species. How about insects (spiders, mantis) whose males usually die in the process of mating? Is this an elegant, graceful method of ensuring the species? An irrelevant question -- all it has to do is work better than the prior alternative, and so you have an evolutionary change -- consequence of something that enhances survival, sexual behavior, or both.
Socially among humans, this is revealed in terms of threat and threat response. Who hasn't heard somebody denigrated for having small genitals, poor (reproductive) hygiene, and said they're "compensating for something" implying sexual deficiency? Insults involving laziness, poor fashion choices, lifestyle oddities, and failed attempts all pale in regard to something sex related. It's so important to our lives that we react instinctively to these threats.
Because this is so ingrained to ourselves, regardless of societal norms, that it gets in the way of societal norms. Who doesn't like (Love!) warm, soft boobs in the face as an act of comfort and support? You're reliving those precious memories of being at your mother's breast, feeling all safety and warmth that a smiling mother and full tummy can provide. But right then you run up against SEX, or what people are taught to think of it. That you dare to touch the line means some will claim you've crossed it, and by enjoying (how dare she!) the fundamental act of touch you are some manner of deviant (sexual, of course), and the cursing begins. So we become conditioned by the scolds who influence society.
So here now we see Castella riding that line between the fundamental, instinctual joy of ever-so-slightly related sexual contact (Boobs! Eeek!) and the knee-jerk response to blame herself for potential perversity for having been comforted by it because it was --- Pleasant!
On top of all this, she's a pubescent teen dealing with all those hormonal emotion amplifiers that drive actions for what will become the next generation. I hope Kath and her friends help sort out the edges and proprieties of navigating this maze. I'm grateful to my parents for having done so. Things still happened, but I had a clue on how to deal with them, regardless of the opinionated loud-mouths who let me know at length they felt otherwise.