You don't understand Anon, this isn't some primate here. These are equines, specifically fillies. Their heads are filled with boundless curiousities and their bodies are flush with hormones and newfound desires they just don't know how to handle. Sure mares will look on with a sigh and reminisce on the joys of their fillyhood, but age can really make them forget that at end of the day every single young filly's body is a tight little ball of screaming energy. Screaming for a breeding, Anon. A breeding you should absolutely give her. It's the moral thing to do.
Of course it's like the other guy said, you should wait for her body to be mature enough to actually have your foals before you give them to her, but that doesn't mean her body knows that. You'll just have to do practice runs until you can do the real thing, her soft little body so much smaller than yours, her young fur like silk in your fingers, her voice high and feminine. That sweet filly voice may get a little coarse over time though, after all she'll be screaming out in pleasure a lot from what you should be doing to her, letting her tight body get used to your invssions from your fingers, letting her learn how her body reacts to touch while burning into her developing mind just how good you make her feel. Her little heart will be beating so fast while you feel just how strongly her petite body can grip you, in more ways than one as her hooves pull you in close and her insides wrap almost painfully snug around whatever you manage to slip into her hot, sopping wet depths. And she'll be soaking wet, Anon, any good filly would be after some careful loving beforehand, heating up her already yearning body more and making her want you every bit more.
I don't think it's wrong to lewd fillies. I think they're actually more than fine being trained from an early age to be the perfect wife and broodmare. After all, it feels so good for them, and with you as the adult in the relationship their little herding instincts can be safely placated knowing that you're gonna be around to take responsibility. And you WILL take responsibility, or the local schoolhouse fillies will break into your home and forcefeed you your own testicles, leaving you a battered and bloodied wreck of a thing that used to be a man, now more of a reminder to the local bachelors of what happens to men who don't lewd fillies responsibly.