As galactic said, those are anecdotes. They do not show a causal relationship, or even a correlation for that matter. In order to demonstrate a causal relation one would need to hypothesize a mechanism for the cause, design an experiment to test the hypothesis, control for confounding effects, etc. While it is not strictly necessary to manipulate the varibles to demonstrate cause, it is extraordinarily difficult to "prove" causal relationships without doing so. And if you wanted to show that early sexual activity caused depression I think you would have a very difficult time getting a human subjects committee to let you run that experiment!
I don't accept at all that depression and risky sexual behavior are interdependent. They might have the same underlying influences but that still does not make them interdependent. A depressed person might engage in risky behavior (sexual or otherwise) out of some kind of self loathing or other emotions but that does not in any way mean that engaging in risky behavior leads to depression (or is even associated with it.) Do you think skydiving, scuba diving, race car driving, or working on undersea oil platforms leads to depression? These are all risky behaviors that many sane and nondepressed people engage in.
Shara, I respect your opinions, and you articulate them well. But I do not agree with you. But I don't have to, nor do we need to restrict discussion to agreeable topics or ones we agree on. Your opinion is as valid as mine.
But referring to the original question posed, I don't think you can blame sexual ethics for the problem of accidental children that a couple cannot support. You and everyone else are free to establish personal ethics and rules of behavior. But ethics for society in general originate from consequences on others or inherent and generally accepted immorality of acts. I think most of us would agree that killing someone is inherently immoral. There are exceptions (self defense, war, capital punishment) that we might debate but in general I think most would agree that the act itself is immoral.
The same cannot be said for sex. Very few (but admittedly some) of us would say the act of having sex is inherantly immoral. Moral considerations with regard to sex stem from consequences. (Many of the prohibitions on sex originated in the middle ages because of confusion over property rights of concubines and illegitimate children, not because fidelity or chastity were considered important!) But we are capable of managing the consequences now through use of birth control, selection of partners, legal policy, and use of measures to prevent transmission of disease.
To me the availability of these modern technologies has made moral and ethical concerns around engaging in sex moot to society at large. Clearly there are concerns for the people involved, but society should butt out.
I do believe there are moral and ethical concerns for society
when irresponsible behavior leads to children that cannot be properly cared for. So getting back to the original poster's question, I think couples don't plan because they are being irresponsible and reckless. It is as simple as that. Their children then suffer for their poor choices and the rest of us often end up paying for it through welfare and other public aid programs.