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.

Of course not.  In freely consenting to the marital embrace without attempting to retard its fecundity through the wife's innate periodic infertility, they have fundamentally submitted to Divine Providence in determining its degree of fruitfulness.  So long as they interiorly assent to the possibility of the blessing of a child with willingness and thanksgiving, I cannot imagine how they could accused of abusing their married state.

What God has made and designed is not evil.  It is the attitude and reasoning behind it that is judged.  There is nothing evil in postponing with grave reason.

The issue is what defines grave reason.  That has been distorted to fit a plethora of scenarios to accommodate convenience even though that has been soundly condemned, even in the conciliar church!

The examples Tiffany gave are clearly grave.  

But inability to financially support?  Not grave.  One or both parents are addicts?  Not grave.  Disruption of the current familial harmony?  Not grave.  Mom's age?  Not grave because if she were truly too old she's couldn't conceive.  Inmaturity on the part of the parents?  Not grave.

You are right songbird that human fertility is good, not evil.  It's human intent that is evil.

Quote from: Quasimodo

If you conceive is it not Gods will? If the baby dies prematurely is it not Gods will? It's up to God if a baby ends up in limbo.

Then is it God's will for babies to be conceived outside of marriage?

A child is a gift from God.

Quote from: Zeitun

Quote from: Quasimodo

If you conceive is it not Gods will? If the baby dies prematurely is it not Gods will? It's up to God if a baby ends up in limbo.

Then is it God's will for babies to be conceived outside of marriage?

I was unaware that we were discussing illicit relationships. Based on the original post I thought we were talking about marriage and the marital act.
God doesn't will that people fornicate. Can life be created against Gods will?

You're talking about God's permissive will.

Everything has to happen by God's permissive will at least, or it wouldn't happen.

But God never wills that we sin.

I agree that the word "grave" needs to be defined.

Didn't Pius XII say something about economic reasons? Or was that meaning in a Depression?

We've had priests (yes, plural) tell us to use NFP because we had children year after year. One even said to use it for 2 years.

I wonder how many children you all have. It's kind of easy to be an armchair referee when you don't have many children, or they are naturally spaced so by the time you have 9, your oldest is in late teens.  The parents now-a-days have way more on their plate than back in the day (homeschooling, no Catholic family support, high costs, etc). NFP could be evil if it's abused. I know, I know...I'm not going to go on about NFP. Not at all. Just a comment I had to share.

My point was that a person sins in fornicating. It is not Gods will that a person fornicates. If God didn't want a life created through this sinful act he wouldn't give it life. Show me proof that people can conceive against Gods (perfect) will.
Does God not will that a baby conceived during rape live? And just so we are clear the rape is obviously not Gods will it is an evil act. Sorry to bring up another example not related to the op.