Are female wild animals stressed all the time?
I don't know the answer as far as stress goes but I do want to mention something. Most animals, unlike humans, have a heat cycle. They only mate during this cycle, which only comes at certain times of the year. In horses, for example, it usually begins in February and they have a heat cycle once a month til about August unless they become pregnant. They only breed a couple of days during this cycle. There is a breeding season for deer for example in the US in the fall for a few weeks. My point is that female animals do not have to fend off males agressively trying to mate all the time. When it is time to mate, during their estrus cycles, the females are usually receptive. They also appear to enjoy caring for their young. Finding food available all the time could be seen as stressful. I think we have to be careful seeing things as a human would, or trying to see animals as in human situations. Some might claim the opposite of you, that animals need to be "free" and not in zoos or even as pets, but should be left in the wild as nature intended in order to be happy.
This of course, is my opinion. No one can know what animals think or feel.