Thank you for all of the answers thus far. I wasn't terribly clear in my original post, but what I was really getting at was the third point I made, not the first two.
If a married couple is doing nothing to avoid having a child, and has a "normal" life when it comes to the frequency of the marital act, but the wife does not get pregnant, am I correct in believing that the couple commits no sin by simply continuing to live as they are? In other words, married couples are under no obligation to make a concerted effort to have more (or any) children, whether that means engaging in the marital act more often, around the time the wife is most fertile, etc.