"Some couples cannot conceive. These are unnatural crosses that God uses for good."

This tracks, God takes all things and leads it towards good. The issue you have in your phrasing following this is what causes a problem, it seems to be a phrasing oversight, since the rest of your article addressing infertility seems to have the message of God using our grief to help edify others. But you revert back to a previous phrase that implies God made the children die to strip away our idols (the children?)
"And at the same time, God promises that all these custom-made crosses, infertility included, will be cured on the Last Day."
That use of "custom-made crosses," only leaves an implication that this is some how previously connected with the act of idolatry.
I have heard LCMS pastors talk on these notes of tragedy, genetic disease, abuse, and illness, and that God ultimately turns all evil toward good before, but I don't think any would agree with the implication that God custom-made a cross for us, to remove idols, that includes birth defects, loss of children in pregnancy or infancy, or suffer acts of violence. God knows the sin in our life that we will experience and commit, he does not create our suffering from sin.