As Faceman has pointed out only those who are ignorant of breeding practices have a problem with this topic. To prove a mare you have to BREED her. Breeding a 3 year old mare does no long term harmful damage to a mare. Period.
Of the 21 mares that I own only 5 of them have not been bred me or my family.
I just purchased an Arabian stallion who is 15 becuase of the foals I have seen that he has sired. I purchased him becuase he will cross well on the mares that I own not the other way around. Has he ever set foot in an arena? No. After seeing the foals that he produced on a wide variety of mares he is well worth his price.
Breeding a horse is not a crime nor should it be. If done right it will produce a good quality animal. Shalom