In a natural world, these animals would have the choice to reproduce (or not). You're forcing them to reproduce and then saying, "Well, it's OK, because they wouldn't exist otherwise." If these animals truly wanted to reproduce in this environment, we wouldn't need to artificially inseminate them.

There are many species that stop reproducing depending on their environment. In zoos, this can be a big problem.

If it were humans in this situation, would you feel the same way? Is your treatment of them excused because you "allowed them to live"? Why is it OK to do this to a pig but not, say, a dog? Or a gorilla?