I feel like its similar to omegas and alphas. Humans being the omegas
He could tell them he is human, but since they are so rare in that universe, he would definitely become a source of interest and might even be bullied for it (imagine how autistic people sometimes go their whole lives pretending to be abled to avoid annoyances). Humans just naturally became scarce, probably due to intermarriage with demons, who had stronger Gene's and so on.
The making out being called "eating" was mostly born bc the uke thought demons would really literally eat him if they found out he is a human (he had this idea bc his brother teased him as a child and he believed him), so when he was kissed he thought it was it, but how he just goes with it as quirk. The seme, being the dense idiot he is, keeps calling it eating bc he can't see he's in love and just kissing his lover.
I'm confused, why can't he reveal that he's a human? It seemed like at the start of the story if anyone found out he's human they'd kill him. Is that what happened to all the other humans? Why do they keep calling making out "eating"? Is that just what they call it? If making out with a human helps your horns not hurt why would you need to hide your human status? Wouldn't that make humans a coveted group?