Calicos are female because both the orange and the black color gene are on the X chromosome, so you need two X chromosomes to get both colors (or something like that, I don't know the specifics). Hence, calicos and tortoiseshells are almost always female as females have two X chromosomes. When you have a male, I believe they are actually a XXY instead of an XY and that's how they get both colors. Also, apparently male calicos are sterile (so good thing Kokonoe appears to be gay).
You know, I read Shingo's story first before this one. And I felt bad for him cuz I thought he got drugged and raped by Keiichi. But then an extra side story showed that Keiichi didn't do it on purpose, and Shingo asked him for sex while drunk off of the silver vine. Even so, I was a bit iffy about that, but now I read this and I'm like, yeah screw you Shingo, you were literally molesting a high schooler. Not that I'm saying I dislike Shingo, I think he's quite cute in his stories. Just saying I don't mind the beginning of his relationship with Keiichi that much anymore lol.
Side Note: Aren't calico cats mostly female? I mean, I guess a male can be calico, but it's quite rare. Kinda fitting of Kokonoe I think.