@luvicvis @Nakanowatari @Goldrop
A competing theory of mine is if Youta(MC) compels the girl to mate through touch or telling them he's male they get a collar. But if they essentially fall for him in someway before knowing he's actually male they don't get the collar.
Lilia at the beginning the first to not get the collar, she masturbated all day, gave him a BJ before sex. But she is also a Tomboy who was always fantasizing about meeting a male and mating by choice. Since in chapter 5 page 14 she is devastated by being turned down by the only male she believes she'll ever meet. https://mangadex.org/chapter/8413/14
The collar is for females he enslaves no collar is for those who come to him freely uncompelled.
If that makes sense.