I'm pretty sure the boy is not actually rejecting the girl's advance because he already got her molt. it's more like in the middle of his confusion and embarrassment for being caught red-handed about his fetish by the source of his fetish no less, he carelessly blurted out that the thing about the girl that excited him the most is her molted off skin.
Judging by the next panel, it's clear the boy regrets his careless remark and want to take on the girl's previous offer. I mean, with him going out with her, he secured a safer and easier way of acquiring her molt.
Tho i do feel bad for the girl, when the boy you're interested in is basically interested more with your excrement than your own self. XD
I guess if you see the molt as just solidified sweat, the boy is basically just a more advanced sweat fetishist?