Let's just say we don't have feral children in the street yet. There aren't that many homeless babies and it is very hard to become adoptive parents. I've heard of couples with fertility issues conceiving before they get an adoptive child.
Another point is that humans have the ability to reason when it comes to the future. We know "Gee, if I get pregnant and I don't have the means to support this baby, I have a problem" So many of us wait till the right time and we don't abandon our children. We seek help is we can't support them and usually wait till thier 18 to boot them out.
Animals can't reason like that. Without natural preditors they will multiply and multiply some more! My boar can't reason that if he "does the deed" that 3-4 babies will show up in 2 months.
Now as thier guardians we are supposed to make the decision for them. We are supoosed to know when to stop, and when breeding is healthy. We breed endangered animals in captivity and I have a hunch that nobody has a problem with breeding animals like rhino's and elephants because we do need more. But there are many guinea pigs, cats, dogs, hamsters that need homes. We don't need anymore. Now I'm not sure about horses. I don't know if there is a horse over population problem or even how hard birth is on the mother so I can't tell you about horses, but I do know that we don't need anymore companion animals to be born into suffering. I don't want to see more kittens and puppies at the Humane Society. What is the point of being brought into this world if you will be taken right out of it with a needle and a pat on the head soon after?