Yes, this logic would apply to animals who are not mature. You're comparison is flawed because children do not want to engage in sex, but clearly animals do. As is demonstrated by this poster, sex with his horse is totally concensual (ie, you cannot physically have sex with a horse unless the horse wants to).
That said, I think you do have a good argument if you made a more accurate comarison. Instead of likening beastiality to paedopheia, you can make a strong case that it is abuse of power to engage in concensual sex with another human if that human is mentally retarted, autistic, or very thick. Or drunk! And on that level, I would agree. I think the point that is lost when discussing this kind of non-consensual sex is that 'true love' cannot exist in these situations because it cannot be reciprocated by the object of affection. And sex without reciprocal love is likened to power play and sexual abuse.