If you want to put health reasons aside for a moment, consider this. How many mares are finished with their training at age 2? 3? 4? 5? Training is a stressful process as is carrying a foal and caring for a foal. I don't think that it is healthy to combine those two stresses. Training can be stalled/negatively affected and/or the foal can be lost. Also, studies in other animals have shown side effects in offspring where the mother has been subjected to stress.
If a mare is intended for the sole purpose of being a broodmare, I have heard of many people breeding at age 3. I don't think that is entirely unreasonable - that is her intended job. Who is to say that is more cruel than putting a horse on a pack line, asking them to strain their legs barrel racing, or any other thing humans ask horses to do.
If a mare is intended to be a riding horse, I would say common sense would suggest that she should not be bred until her training is complete. I have followed similar guidelines for my stallion. He is five this year, and we just now bred him to our mare because I was comfortable with his training level (still far to go to refine skills in various disciplines, but very strong foundation). I am not opening him to stud until next year, and even then I am limiting it. Why? Because his primary purpose is as a riding horse, secondary purpose is as a breeding horse. Horses have many years they can breed; if you are not a full on breeder, what is the rush?
We have a mare that we bought last year at age 7. She had been bred by a previous owner at age two. She is beautiful, gentle, kind, and healthy. She knows all of the basics of riding, and has some foundation in cutting, but I think her skills are a little unrefined for a now eight year old mare. Did breeding at two change her in any way? I don't know - I would not choose to do that - but I don't know
As for teenage humans getting pregnant - it is very sad - they are loosing their own youth and carefree years. Why? Are they to blame? I don't think that is necessarily 100% the case. They of course must take responsibility for their actions, but what about the parents, schools, society that keeps them so uninformed that they may not really understand the full consequences of their actions. I am not a parent, but parents out there - would you prefer to keep your children in the dark about contreceptive the birds and bees because you don't condone it at their age based on you religion or other morals OR would you prefer your child is educated and prepared enough that you feel confident that they can make a smart decision on their own. I know that many kids will find a way to do what their hormones and often peers are driving them to do regardless of how hard their parents try to protect them.
I wouldn't say disgusting, I agree that life is a wonderful thing, but I think many teenagers would have had made better parents if they were allowed to finish of their youth and learn better the ways of the world and what is important to them, etc.