Caliban said:
In general, young children aren't sexual, and thus aren't effective as symbolic sex objects. They are good as symbolic sacrifices for other purposes though (such as youth, unrealized potential, or innocence.)
You would want an adolescent virgin, to release the maximum amount of repressed sexual energy when you sacrifice them.
Attractive young women serve as a nexus of sexual energy. (Because of the males directing their unrequited sexual and mental energies toward them. All those guys fantasizing or obsessing about her creates a buildup of psychic energy.)
You don't get nearly the same potency when sacrificing an ugly virgin (although they will serve in a pinch).
In general, a male won't have as many females infatuated with him as a female would have males infatuated with her. Also, males tend not to remain virgins long if they do have several females lusting after them, while social mores make it more likely that females, even attractive females, will remain a virgin long enough for you to track them down and sacrifice them.
Females are preferred as sacrifices because women symbolize all the powers of creation, life, fertility, and birth. It's that whole womb thing.
...What?
Caliban, have I mentioned you are a Genius.